docs: SRS crash course section

[polarathene]

It seems that the main README with 2.x release no longer includes the SRS overview:

https://github.com/roehling/postsrsd/blob/1.x/README.md#sender-rewriting-scheme-crash-course

Was that intentional, or does it live somewhere else?

[roehling]

It was intentional because I thought it was not really needed by PostSRSd users. I can re-add it, though, if there is demand.

[polarathene]

I only noticed because a third-party contribution to docs for a project I maintain updated a link to the v2 README where this section was missing.

It was linked as context for how SRS worked (which users that seek out PostSRSd probably already know, but in our project we bundle it as a feature that can be toggled as opt-in), so it depends if that extra bit of insight is worthwhile. I'm sure I could just direct the reader to an alternate resource elsewhere too 👍