docs: Add guidelines for contributing

This patch adds a document with guidelines for contributing to chasquid.

It includes suggestions for how to ask questions, how to send patches
(and the expectations around them), and documents how the different
branches are used.

Thanks to raspbeguy (https://github.com/raspbeguy) for suggesting this
improvement.

.mkdocs.yml

@@ -16,6 +16,10 @@ markdown_extensions:
 
 theme: readthedocs
 
+exclude_docs: |
+  man/*.1
+  man/*.5
+
 nav:
 - Home: index.md
 - How-to: howto.md
@@ -35,4 +39,9 @@ nav:
     - tests.md
     - relnotes.md
     - knownissues.md
+    - contributing.md
+
+not_in_nav: |
+  security.md
+  man/*