Track the xml2rfc dependency using a requirements.txt file

The standard practice for tracking dependencies on Python projects is to
declare the dependencies and the expected versions on a
`requirements.txt` file that is read by `pip`.

Right now, we request people to manually install the latest version of
`xml2rfc`. After this commit, we properly track `xml2rfc` on
`requirements.txt` and tweak the documentation a bit to show users how
to install it from there instead.

Signed-off-by: Juan Cruz Viotti <jv@jviotti.com>

Author: jviotti

README.md

@@ -35,7 +35,14 @@ Labels are assigned based on [Sensible Github Labels](https://github.com/Releque
     * links.json - JSON Hyper-Schema's Link Description Object meta-schema
     * hyper-schema-output.json - The recommended output format for JSON Hyper-Schema links
 
-Install "xml2rfc" using "pip" (https://pypi.org/project/xml2rfc/) and type "make" at a shell to build the .txt and .html spec files.
+Install "xml2rfc" using "pip" (https://pypi.org/project/xml2rfc/) and type "make" at a shell to build the .txt and .html spec files:
+
+```sh
+pip install --requirement requirements.txt
+make 
+```
+
+The version of "xml2rfc" that this project uses is updated by modifying `requirements.in` and running `pip-compile requirements.in`.
 
 Descriptions of the xml2rfc, I-D documents, and RFC processes:
 

requirements.in

@@ -0,0 +1 @@
+xml2rfc>=3.12.0

requirements.txt

@@ -0,0 +1,55 @@
+#
+# This file is autogenerated by pip-compile with python 3.9
+# To update, run:
+#
+#    pip-compile requirements.in
+#
+appdirs==1.4.4
+    # via xml2rfc
+certifi==2022.6.15
+    # via requests
+charset-normalizer==2.1.0
+    # via requests
+configargparse==1.5.3
+    # via xml2rfc
+google-i18n-address==2.5.2
+    # via xml2rfc
+html5lib==1.1
+    # via xml2rfc
+idna==3.3
+    # via requests
+intervaltree==3.1.0
+    # via xml2rfc
+jinja2==2.11.3
+    # via xml2rfc
+kitchen==1.2.6
+    # via xml2rfc
+lxml==4.9.1
+    # via xml2rfc
+markupsafe==2.1.1
+    # via jinja2
+pycountry==22.3.5
+    # via xml2rfc
+pyflakes==2.4.0
+    # via xml2rfc
+pyyaml==6.0
+    # via xml2rfc
+requests==2.28.1
+    # via
+    #   google-i18n-address
+    #   xml2rfc
+six==1.16.0
+    # via
+    #   html5lib
+    #   xml2rfc
+sortedcontainers==2.4.0
+    # via intervaltree
+urllib3==1.26.10
+    # via requests
+webencodings==0.5.1
+    # via html5lib
+xml2rfc==3.12.0
+    # via -r requirements.in
+
+# The following packages are considered to be unsafe in a requirements file:
+# setuptools