Updated README for developers

* Reference the correct files for upcoming changes.

Signed-off-by: Shourya Shukla <periperidip@gmail.com>

Author: periperidip

README.md

@@ -1,2 +1,6 @@
 # su2code.github.io
 [Link to Website](https://su2code.github.io/)
+
+## For Developers
+
+In order to make any changes in the documentation of SU2, the files in [_docs_v7/](_docs_v7) should be touched. This is the documentation which is currently online.