Create/link app guide for RNW version update best practices

[chrisglein]

We hit this issue in triage. I wanted to point them to our upgrade guide, but... either I couldn't find it (link needs to be surfaced more obviously) or it doesn't exist. Our recommendation is basically that you create a new blank project and copy over your app content; that way gets you the cleanest app template. I swear I've seen that written done somewhere, but I couldn't find it.

We may not need to necessarily create new content here, just link to it. For core RN upgrade issues support is available here.

[chiaramooney]

Dupe of #533