Readme: example + an FAQ

[rauschma]

Two suggestions for the readme:

• Show an example of matchAll() in use.
• Answer the question: Why not RegExp.prototype.execAll()? It seems more like a batch version of exec() than a batch version of match() (which already has a batch mode).

[ljharb]

Thanks, great suggestion to show an example!

exec mutates the regular expression, and matchAll will not. match doesn't have a batch mode that includes nested capturing groups.

PRs are welcome for more explanatory text in the interim until I can get to improving it :-)