add a nav link to contributing doc

Signed-off-by: Ramkumar Chinchani <[email protected]>

Author: Ramkumar Chinchani

mkdocs.yml

@@ -117,6 +117,7 @@ nav:
     - Code Structure: developer_guide/code_structure.md
     - LXC Usage: developer_guide/lxc_usage.md
     - OverlayFS Usage: developer_guide/overlayfs_usage.md
+    - Contributing: developer_guide/CONTRIBUTING.md
   - License: license.md
 extra_css:
-  - stylesheets/custom.css
+  - stylesheets/custom.css