Update README to indicate automatic changes

Author: ethomson

README.md

@@ -1,5 +1,8 @@
 # [libgit2's web site](http://libgit2.org/)
 
+
+**Note:** The reference documentation is automatically generated from the in-line documentation in libgit2's public header files. This generated content is pushed from updates to the [libgit2](https://github.com/libgit2/libgit2) repository itself. (Do not change content in the automatically generated directories.)
+
 ```bash
 bundle install
 bundle exec jekyll serve --watch