Clarify that minimal escaping is done (#30) and describe edge cases

[slevithan]

There have been multiple requests to escape additional characters, so this makes it clearer that minimal escaping is done, per #30 (comment). It also gives a solution for more involved context-aware escaping that handles numerous edge cases, for situations when developers need to insert an escaped string at an arbitrary position in a regex.

Fixes #30

[slevithan]

If you would prefer that I take out the link to the regex library (which provides a solution for the numerous edge-case context issues when escaping, and is based on discussion amongst TC39 members about alternative approaches for regex escaping), I would be happy to do so. However, I think it's worth letting people know that such edge cases exist.

escape-string-regexp's lack of support for context-related issues is a reasonable tradeoff in favor of simplicity.

[sindresorhus]

Nice work on the regex package 👍