docs: add startLTS usage examples (#654)

I reach to `node-core-utils` for automating the LTS release commit but I
couldn't get it to work due to the lack of a proper usage example.

I'm adding examples to both the markdown docs and the cli help output which are
the places I would have loved to have found these.

Signed-off-by: Ruy Adorno <[email protected]>

Author: ruyadorno

components/git/release.js

@@ -36,7 +36,9 @@ export function builder(yargs) {
       describe: 'Version number of the release to be prepared or promoted'
     })
     .example('git node release --prepare 1.2.3',
-      'Prepare a new release of Node.js tagged v1.2.3');
+      'Prepare a release of Node.js tagged v1.2.3')
+    .example('git node --prepare --startLTS',
+      'Prepare the first LTS release');
 }
 
 export function handler(argv) {

docs/git-node.md

@@ -233,6 +233,11 @@ git node release --prepare 1.2.3
 git node release --prepare
 ```
 
+```sh
+# Prepare the first LTS release for a given release line
+git node release --prepare --startLTS
+```
+
 ## `git node sync`
 
 Demo: https://asciinema.org/a/221230