Need documentation for snippet support

[jsumners]

PR #22 added support for Alfred's snippet feature, but we didn't include documentation for it in the readme. While researching how the feature may interact in #121, I learned that we don't have that documentation. This should be rectified.

Below is a good summary of the feature. It should be expanded for the documentation.

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For the snippet trigger, you must:

1. First, configure Alfred Snippets (and enable accessibility controls).
2. Configure a snippet expansion for this workflow by editing Snippet input
3. Run the snippet by typing your chosen prefix + snippet keyword

Originally posted by @rmm5t in #22 (comment)