Auto merge of #199 - derekdreery:macro_doc, r=jdm
Auto merge of #199 - derekdreery:macro_doc, r=jdm

Allow the generated macros to have documentation

This PR provides functionality to add documentation to the macros created using `string_cache_codegen`.

It also copies the example from the README to the crate root docs.

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servo/string-cache