Added yarn for installing Cerebral 2 edge

[hipertracker]

Added simpler yarn. It's already used by default in create-react-app, so it makes sense to use it also here.

[coveralls]

Coverage remained the same at 88.873% when pulling c2f6b66 on hipertracker:patch-1 into ec8ba3a on cerebral:master.

[henri-hulski]

Yarn is optional in CRA.
And Yarn didn't work for me (yarnpkg/yarn#746).
It also has some more issues: facebook/create-react-app#1189.

So I think we should at least document both npm and yarn and make yarn optional.

[Guria]

yarn users knows how to substitute npm commands in instructions.
Moreover, I have tested it result sligtly changed in part what we get in package.json. We have to instruct users to have exact versions without range and tag names in their package.json.