Add :require-macro in core.cljs

[tggreene]

In case it's generally useful, I found this worked well for me and makes it straightword to run in a variety of shadow-cljs projects.

This seems to make it more straightforward to use via cljs, although it contradicts the experience of @lucywang000 here https://github.com/weavejester/hashp/pull/7#discussion_r387446427

I've modified the README to suit with an alternative method of running that allows you to use without modifying project configuration.

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