gitignore: Only ignore the README file at the root of the repository
gitignore: Only ignore the README file at the root of the repository

We should be able to modify/add README files else where in the tree and
see them with git status, eg build/mingw/README.

Reviewed-by: Neil Roberts <822ec91a7ef82248fd36162ca8ac3283eba70848@linux.intel.com>

gcampax/cogl,djdeath/cogl,gcampax/cogl,djdeath/cogl-android,djdeath/cogl,djdeath/cogl-android,djdeath/cogl,gcampax/cogl,gcampax/cogl,djdeath/cogl-android