Add doc "how to install gsk on initialized git remote"

[ryuran]

Sometimes git remote is initialized with a first commit of a readme.md.
The installation process described in readme.md can't be used is this case.

[stephane-tessier]

Before documenting maybe we should agree on a procedure ?
One suggestion could be: (I didn't test)

git clone [email protected]:cleverage/garden-starter-kit.git tmp
rm -rf tmp/.git
mv tmp/* .
rmdir tmp

[ryuran]

master is usualy protected with a first commit of readme.md.

Create a new branch and replace anything with gsk content seems to be the best way.
Then commit, push and merge/pull request.