README: explain how the algorithm works

[gasche]

Fixes #8

[waldyrious]

I think it would be useful to add a link to https://github.com/tummychow/git-scripts/blob/master/git-absorb.md#interlude-patch-theory at the end of this section. For example:

For additional details, see the documentation here.

[gasche]

I'm not sure; this document is way too long and possibly outdated (for example it refers to an old implementation). I prefer to keep the proposed change as-is, will change if there is more feedback in favor.

[waldyrious]

Sure, I can see your point. 👍

[tummychow]

yeah okay this is an acceptable summary. i agree that the old documentation shouldn't be referenced here, that form of commutation just does not work