Add a README file to each package

[afshin]

We should have at least a basic REAMDE.md file in each Lumino package. In addition to helping new contributors navigate this mono-repo, it also allows the NPM page for each Lumino package to display some useful information to consumers of the package.

[gabalafou]

As far as I can tell, none of the packages currently have a README.md. If somebody could make a README for one package, I could use that as a template to make more for other packages.

[fcollonval]

Something like the following would be nice

---

@lumino/application

A Lumino package to create web application composed of plug-able plugins.

By default, it contains a CommandRegistry using @lumino/commands, a ContextMenu and a view called shell that must inherits from Widget class (the two later classes are defined in @lumino/widgets).

[gabalafou]

Thanks for the template @fcollonval, I'll work on extending this template to all of the Lumino packages. I don't have permission to assign this task to myself. Could you assign it to me? Thank you!

[afshin]

@gabalafou I've added you to the "Triage" team for the jupyterlab GitHub org, you should be able to assign and label things, etc., now

[gabalafou]

Just a heads up that I won't be able to work on this until next week.

[gabalafou]

I'm punting on this for another week because I wanted to finish #332.