Add contribution guidelines

- Coding style
- How to send patches

Signed-off-by: Michael Opdenacker <michael.opdenacker@bootlin.com>

README.adoc

@@ -319,7 +319,9 @@ and 2 GB for the git repository.
 * We're using an LXD instance with 8 GB of RAM on a cloud server with 8 CPU cores
 running at 3.1 GHz.
 
-= Supporting a new project
+= Contributing to Elixir
+
+== Supporting a new project
 
 Elixir has a very simple modular architecture that allows to support
 new source code projects by just adding a new file to the Elixir sources.
@@ -411,6 +413,16 @@ new project:
 
 You can then check that Elixir works through your http server.
 
+== Coding style
+
+If you wish to contribute to Elixir's Python code, please
+follow the https://www.python.org/dev/peps/pep-0008/[official coding style for Python].
+
+== How to send patches
+
+The best way to share your contributions with us is to https://github.com/bootlin/elixir/pulls[file a pull
+request on GitHub].
+
 = REST api usage
 
 After configuring httpd, you can test the api usage: