Add new page for governance info

[crandmck]

This is for #632. I created a new page "Contributing to Express" and copied into it info from 3 files in expressjs/express:

• https://github.com/expressjs/express/blob/master/Collaborator-Guide.md

• https://github.com/expressjs/express/blob/master/Contributing.md

• https://github.com/expressjs/express/blob/master/Security.md

  I also consolidated some info in this repo that was duplicated between README.md and CONTRIBUTING.md.

[crandmck]

@expressjs/express-tc PTAL.
We might want to add some links to other docs in the Node Foundation. Open to suggestions.

[blakeembrey]

I think we should change strongloop to expressjs now.

[crandmck]

Good catch @blakeembrey I made that change. I'll open a separate PR to change https://github.com/expressjs/express/blob/master/Collaborator-Guide.md which has the same mistake.

[dougwilson]

Looks good overall.

[crandmck]

I'll go ahead and merge, and we can iterate further if desired.