doc: add LTS info to COLLABORATOR_GUIDE.md

There is currently no information in the Collaborators guide regarding
LTS. This commit adds some basic copy explaining what LTS is, what is
considered for LTS, and a simple way collaborators can help.

Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Steven R. Loomis <srloomis@us.ibm.com>
PR-URL: https://github.com/nodejs/node/pull/3442