Add a recipe demonstrating how to create user keywords

[adrianyorke]

We don't currently have any recipes with *** Keywords *** section/table that demonstrate how to create your own user keywords.

For example, in section 9 we could rewrite Recipe 091 Working With Requests Library so we have new keyword Check Github User Exists and Check Github User Does Not Exist

The test case Get Request test case could be rewritten to check several accounts that do exist and several accounts that should not exist.