Added Documentation for creating downloader plugins

Documentation related to creating plugins.

Author: sean-morris

README-PLUGINS.md

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+# nbgitpuller - downloader plugin documentation
+
+nbgitpuller uses [pluggy](https://pluggy.readthedocs.io/en/stable/) as a framework
+to load any installed nbgitpuller-downloader plugins. There are three downloader plugins
+available right now:
+- [nbgitpuller-downloader-googledrive](https://github.com/jupyterhub/nbgitpuller-downloader-googledrive)
+- [nbgitpuller-downloader-dropbox](https://github.com/jupyterhub/nbgitpuller-downloader-dropbox)
+- [nbgitpuller-downloader-generic-web](https://github.com/jupyterhub/nbgitpuller-downloader-generic-web)
+
+
+There are several pieces to be aware of for the plugin to work correctly:
+1. The setup.cfg(or setup.py) file must have the entry_points definition.
+For example:  
+> [options.entry_points]  
+nbgitpuller =  
+      dropbox=nbgitpuller_downloader_dropbox.dropbox_downloader
+
+2. The file referenced for use by nbgitpuller in the plug-in (the above example is looking for the 
+file, dropbox_downloader) must implement the function handle_files(query_line_args) and be decorated with `@hookimpl`.
+3. As a consequence of this, the following must be imported:
+    - `from nbgitpuller.hookspecs import hookimpl`
+4. The implementation of the handle_files function in your plugin needs to return
+   two pieces of information:
+   - the name of the folder, the archive is in after decompression
+   - the path to the local git repo mimicking a remote origin repo
+   
+nbgitpuller provides a function in plugin_helper.py called handle_files_helper that handles the downloading
+and returning of the correct information if given a URL, the extension of the
+file to decompress(zip or tar) and the progress function(I will describe that
+more later) but you are welcome to implement the functionality of handle_files_helper in your
+plug-in. There may be use cases not covered by the currently available plugins like needing to authenticate against
+the webserver or service where your archive is kept. Either way, it behooves you
+to study the handle_files_helper function in nbgitpuller to get a sense of how this function
+is implemented.
+
+For the rest of the steps, I refer you to the [nbgitpuller-downloader-dropbox](https://github.com/jupyterhub/nbgitpuller-downloader-dropbox) plugin.  
+   ```
+   @hookimpl  
+   def handle_files(query_line_args):
+      query_line_args["repo"] = query_line_args["repo"].replace("dl=0", "dl=1")  # dropbox: dl set to 1  
+      ext = determine_file_extension(query_line_args["repo"])`  
+      query_line_args["extension"] = ext
+      loop = asyncio.get_event_loop()
+      tasks = handle_files_helper(query_line_args), query_line_args["progress_func"]()
+      result_handle, _ = loop.run_until_complete(asyncio.gather(*tasks))
+      return result_handle
+   ```
+
+The following pieces describe what happens in handle_files before, at least, in this case, we call
+the handle_files_helper function:  
+
+1) The parameter, query_line_args, is all the query line arguments you include on the nbgitpuller link. This means you 
+   can put keyword arguments into your nbgitpuller links and have access to these arguments in the handle_files
+   function.   
+   For example, you might set up a link like this:   
+   http://[your hub]/hub/user-redirect/git-pull?repo=[link to your archive]&keyword1=value1&keyword2=value2&provider=dropbox&urlpath=tree%2F%2F
+   In your handle_files function, you could make this call to get your custom arguments:
+
+   ```
+   query_line_args["keyword1"]
+   query_line_args["keyword2"]
+   ```
+2) The query_line_args parameter also includes the progress function used to monitor the download_q
+   for messages; messages in the download_q are written to the UI so users can see the progress and 
+   steps being taken to download their archives. You will notice the progress function is passed into 
+   handle_files_helper and accessed like this:
+   ```
+    query_line_args["progress_func"]
+    query_line_args["download_q"]
+   ```
+3) The first line of the handle_files function for the dropbox downloader is specific to DropBox. The URL to a file
+   in DropBox contains one URL query parameter(dl=0). This parameter indicates to Dropbox whether to download the
+   file or open it in their browser-based file system. In order to download the file, this parameter
+   needs to be changed to dl=1. 
+4) The next line determines the file extension (zip, tar.gz, etc).
+   This is added to the query_lines_args map and passed off to the handle_files_helper to
+   help the application know which utility to use to decompress the archive -- unzip or tar -xzf.
+5) Since we don't want the user to have to wait while the download process finishes, we have made
+   downloading of the archive a non-blocking process using the package asyncio. Here are the steps:
+    - get the event loop
+    - setup two tasks:
+        - a call to the handle_files_helper with our arguments
+        - the progress_loop function
+    - execute the two tasks in the event loop.
+6) The function returns two pieces of information to nbgitpuller:
+    - the directory name of the decompressed archive
+    - the local_origin_repo path.
+
+The details of what happens in handle_files_helper can be found by studying the function. Essentially, the archive is downloaded, decompressed, and set up in a file
+system to act as a remote repository(e.g. the local_origin_repo path).
+
+