Move steps/process documentation over to nodejs/node

[BethGriggs]

In a similar vein to #522 - As discussed in the previous WG meetings, move Releaser process steps over to Releases.md nodejs/node.

So far the only section I could identify as a candidate to be moved to Releases.md was https://github.com/nodejs/Release#marking-a-release-line-as-lts

• Remove https://github.com/nodejs/Release#marking-a-release-line-as-lts from Node.js Release README
• Add to https://github.com/nodejs/node/blob/master/doc/releases.md as a new heading (probably before the "Major Releases" heading)

//cc @danielleadams as I believe you volunteered for this