Document how to apply Ruby patches
What does this Merge Request do?
Related to gitlab#296530 (closed).
This documents how to patch GDK when using asdf to apply custom Ruby patches.
Merge Request checklist
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Tests added for new functionality. If not, please raise Issue to follow-up. 
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This change is backward compatible. If not, please include steps to communicate to our users. 
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Documentation added/updated, if needed. 
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gdk doctortest added, if needed.
Edited  by Kamil Trzciński