Clarify Github Gists explanation

[benblasco]

Thank you for creating this great extension. I propose we clarify the explanation of using Github gists to make it clear that the Gist ID is (for example) "f545156c0083f7eaefa44ab69df4ec37" rather than "https://gist.github.com/username/f545156c0083f7eaefa44ab69df4ec37". Happy to do this via a PR if you are comfortable with a stranger's contributions! I can even update the explanation with more detailed screenshots if you like.

[oae]

Added some guides on the readme. Thanks for the feedback