No documentation for dealing with unusual Github release tags

Some projects (i.e, vectorscan, suricata) use release tagging that isn't purely semver tagging. This causes gh() to break. Documentation was provided by @Owen-sz for how to work around this using string manipulation but this isn't made clear for people working with rhai for the first time.

Start with the documentation covering gh() and the existing guidance provided by Owen-sz. Document how to handle release tags that are not purely semver, including the string-manipulation workaround, so someone new to rhai can find and follow it.

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