contributions.md Add doc about how to contribute to flatbuffers (#8464)

docs/README.md

@@ -0,0 +1,23 @@
+# Documentation
+
+This is the source of the FlatBuffers documentation, that is served at
+https://flatbuffers.dev.
+
+
+## Local Building
+
+The documentation can be built and served locally during development, see [https//flatbuffers.dev/contributing/#local-development] for full details.
+
+__tl;dr__
+
+Install:
+
+```
+pip install mkdocs-material
+```
+
+Build and Serve:
+
+```
+mkdocs serve -f docs/mkdocs.yml
+```