de-dupe readme and finalise docs site (#441)

* save old readme for migration purposes

* promote new readme

* use the repo readme from the doc site homepage

* mention extendable plugin system in title and old plugin support

* improve subtitles english

* update links to direct users to the site!

* add link to docs site in readme until github repo site link is populated

* update troublesome contributing section

CONTRIBUTING.md

@@ -20,7 +20,7 @@ details like asdf version, shell, OS, etc...
 ## Did you write a patch that fixes a bug?
 
 Open a new GitHub pull request with the patch. Refer to the [Development
-section of the README](https://github.com/asdf-vm/asdf#development) for the
+section of the README](http://asdf-vm.github.io/asdf/#/contributing-core-asdf-vm) for the
 details on how to run the unit tests. Please make sure that unit tests pass on
 Travis CI.