Add brief comparison to other in-memory cache libraries

[erni27]

Add a new README section with brief comparison between imcache and other in-memory cache libraries.

It would help users looking for an in-memory cache library to better understand if imcache is a great fit for them.

[GreyXor]

Compare with https://github.com/zekroTJA/timedmap (among others), seems quite similar but is not generic.

[erni27]

@GreyXor: Thanks! Will take a look when I finally sit down and do it.