Promote aurelia-typed-observable plugin to official and add docs

Just today I again stumbled across this great plugin https://github.com/aurelia-contrib/aurelia-typed-observable-plugin which definitely deserves more visibility. How about promoting it to an official one and adding some docs @EisenbergEffect, @bigopon ?

Review the aurelia-contrib/aurelia-typed-observable-plugin repository linked in the issue and the surrounding discussion here. Determine whether the plugin should become an official Aurelia plugin and what documentation is expected; done would require agreement on promotion and completed documentation, but no target files or tests are identified.

Written by the indexing model from the issue text.