Add a Readme note about adding the gem to the top of Gemfile

[ci skip]
Other gems may use `ENV` vars for initializing their settings.
To ensure they use the correct values, `dotenv-rails` should appear at the top of the Gemfile to be loaded first.
Merge pull request #81 from RKushnir/patch-1

Add a Readme note about adding the gem to the top of Gemfile