Update github documentation

[Robert_Jensen1]

Please update your documentation on GitHub -- it's terribly out-of-date and is full of broken links. This makes any development for newcomers very frustrating and difficult to follow. E.g. roadmaps and changelogs haven't been updated in over a year. Please either maintain the page or redirect it to something that is maintained: https://github.com/Microsoft/PowerBI-visuals/blob/master/ChangeLog.md was last updated on Apr 24, 2017 and points to https://github.com/Microsoft/PowerBI-visuals/blob/master/Roadmap/README.md (which was last updated on Dec 20, 2017)

[fbcideas_migusr]

Even more frustrating is that the following migration guide to pbi visuals tools 3.0 keeps on pointing to a branch that don't exist, resulting in 404 errors whenever it tries to reference a diff... https://microsoft.github.io/PowerBI-visuals/docs/how-to-guide/migrating-to-powerbi-visuals-tools-3-0/ routinely points to branch "sample-next", but only the following branches exist: LandingPage newContextMenu contextMenuSupported dev/next master