x/website: suggestion to define or explain what CL is in contribute.html

[jfussion]

We've replaced CL with change in https://go-review.googlesource.com/c/go/+/36564/ to not confuse people what it means but CL is everywhere and I don't see any definition of that.

We may want to define or explain what CL is in contribute.html since we use it to refer to a PR or a change (at least that's how I understand it base on the CL linked above).

[dmitshur]

/cc @andybons @jayconrod

[seankhliao]

I believe the first instance of CL now has the full name: "Gopherbot will create a corresponding Gerrit change list (a "CL")"