Update install.rst

Missing mention about "migrations" folder and init command which creates your phinx.yml config file.
docs: rephrase sentence on auto-incrementing column

I didn't understand what that sentence was saying until I actually try to add an `id` column, this should be clearer.
Merge pull request #311 from maxgalbu/patch-1

docs: rephrase sentence on auto-incrementing column
improve copy
Merge pull request #295 from hasannawaz/patch-1

Update install.rst
Merge branch '0.3.x-dev' of github.com:robmorgan/phinx into 0.3.x-dev