Create an FAQ

We should create a helpful document of common questions, issues and corresponding solutions. For example the error https://github.com/dart-lang/webdev/issues/375 can be caused by a user's corrupted environment. Action must be taken by the user to resolve. This should be documented and easily accessible.

cc @kevmoo

Review issue #375 first, then inspect the repository's existing documentation entry points to decide where an FAQ belongs. Document the common questions and solutions described in this issue, including the corrupted-environment case, and make the finished FAQ easy to find.

Written by the indexing model from the issue text.