README instructions always point to GN_VERSION=master

[nicokant]

The instructions on the README should be improved and should be clear that GN_VERSION is the actual branch of geonode-project you want to set up.

I noted that because I was setting up the project following the instructions from the branch 4.2.x but I got installed the master branch

[giohappy]

Thanks @nicokant Done in https://github.com/GeoNode/geonode-project/commit/a09070e612d93c811497c2ede9793bf3edcbefc5