initial doc contrib page

doc/source/contributing_docs.md

@@ -8,6 +8,9 @@ code within gstlal repo, since the documentation files are updated by a similar
 In general, the gstlal documentation uses [RestructuredText (rst)](https://docutils.sourceforge.io/rst.html) files
 ending in `.rst` or [Markdown](https://www.markdownguide.org/basic-syntax/) files ending in `.md`.
 
+The documentation files for gstlal are located under `gstlal/doc/source`. If you add a new page (doc file), make sure to
+reference it from the main index page.
+
 Useful Links:
 
 - [MyST Directive Syntax](https://myst-parser.readthedocs.io/en/latest/syntax/syntax.html#syntax-directives)