Tracker: Update every README

[sethladd]

some of the READMEs in this repo point to outdated places to get more help (such as gitter), and some READMEs tell folks to create issues in flutter/flutter.

We should visit every README and remove all the outdated "to get help" links at the bottom, and normalize all the links to the appropriate issue trackers, which is presumably flutter/samples/issues

(ewindmill edit) Each README should clearly state what the reader can find in that sample