Add CONTRIBUTING.md (#2121)

Author: cruessler

CONTRIBUTING.md

@@ -0,0 +1,26 @@
+# Contributing
+
+We’re glad you found this document that is intended to make contributing to
+GitUI as easy as possible!
+
+## Building GitUI
+
+In order to build GitUI on your machine, follow the instructions in the
+[“Build” section](./README.md#build).
+
+## Getting help
+
+There’s a [Discord server][discord-server] you can join if you get stuck or
+don’t know where to start. People are happy to answer any questions you might
+have!
+
+## Getting started
+
+If you are looking for something to work on, but don’t yet know what might be a
+good first issue, you can take a look at [issues labelled with
+`good-first-issue`][good-first-issues]. They have been selected to not require
+too much context so that people not familiar with the codebase yet can still
+make a contribution.
+
+[discord-server]: https://discord.gg/rZv4uxSQx3
+[good-first-issues]: https://github.com/extrawurst/gitui/issues?q=is%3Aissue+is%3Aopen+label%3A%22good+first+issue%22

README.md

@@ -38,7 +38,8 @@
 11. [Key Bindings](#bindings)
 12. [Sponsoring](#sponsoring)
 13. [Inspiration](#inspiration)
-14. [Contributors](#contributors)
+14. [Contributing](#contributing)
+15. [Contributors](#contributors)
 
 ## 1. <a name="features"></a> Features <small><sup>[Top ▲](#table-of-contents)</sup></small>
 
@@ -276,12 +277,16 @@ The key bindings can be customized: See [Key Config](KEY_CONFIG.md) on how to se
   - It would be nice to come up with a way to have the map view available in a terminal tool
 - [git-brunch](https://github.com/andys8/git-brunch)
 
-## 14. <a name="contributors"></a> Contributors <small><sup>[Top ▲](#table-of-contents)</sup></small>
+## 14. <a name="contributing"></a> Contributing <small><sup>[Top ▲](#table-of-contents)</sup></small>
+
+See [CONTRIBUTING.md](CONTRIBUTING.md).
+
+## 15. <a name="contributors"></a> Contributors <small><sup>[Top ▲](#table-of-contents)</sup></small>
 
 Thanks goes to all the contributors that help make GitUI amazing! ❤️
 
 Wanna become a co-maintainer? We are looking for [you](https://github.com/extrawurst/gitui/issues/2084)!
 
 <a href="https://github.com/extrawurst/gitui/graphs/contributors">
   <img src="https://contrib.rocks/image?repo=extrawurst/gitui" />
-</a>
+</a>