Separate changelog and README, add basic how-to in README

[aurelient]

The README is mostly a changelog at the moment.

A short README explaining how to get started (files to use depending on the journal/conference, which flavour of Latex works best, etc.) would be super appreciated!

PS: thanks for all the work on the new templates

[stephennspencer]

The LaTeX User Guide is available from https://www.acm.org/publications/proceedings-template and should answer your questions. (This document makes clear that what is available in the Github repo is a development version of the template.)