Create a README.md

[jasvir]

This is mostly a copy of the introductory text on the old Google developers site with a link pointing back there for the rest of the documentation.

[kpreid]

I recognize that this isn't new text, but it really needs updating. Caja is not a "compiler" any more, and ES5 and strict mode are no longer things we emulate but things we require.

Also, please reformat the commit message: header, review, blank, description wrapped at 72, blank, reviewer.

Create a README.md
https://github.com/google/caja/pull/1987

This is mostly a copy of the introductory text on the old Google
developers site with a link pointing back there for the rest of the
documentation.

R=kpreid@google.com

Also, developers.google.com isn't "old". Perhaps you're thinking of code.google.com!

[teppeis]

@jasvir @kpreid created by #2028