Add TODO with some notes.

TODO.md

+# TODO
+
+## Documentation structure
+
+* Where should we store deprecated documentation? Should we include it at all?
+* Should we rename `develop.md` (`core/develop.md` vs `extension/develop.md`?)
+
+## Extension questions
+
+* How do I include Composer libraries in my CiviCRM extension?
+* Should I add the `vendor/` directory to Git?
+* How do I release my extension?
+* How do I provide downloadable .zip files from Github (which are working builds)?