should update README with "how to develop" section

[hoatle]

$ cd ~/
$ git clone <your_forked_repo> teracy-dev
$ cd teracy-dev
$ git remote add upstream git@github.com:teracyhq/dev.git
$ git fetch upstream # get the changes from the upstream repo
$ git checkout develop # the develop branch to work on it
$ git branch -u upstream/develop # track the upstream's branch
$ git pull # update to the latest upstream's changes