Add instructions for running Flask with PyCharm

[davidism]

With the introduction of the flask command, setting up PyCharm is not as straightforward. Some screenshots and instructions would be useful. Similar to the section on how to deploy to common platforms.

[ThiefMaster]

Maybe PyCharm should have some shortcut to run an app tthrough the flask cli since they already have claim to support flask? It'd probably be a good idea to suggest this in their issue tracker.

[pmayer]

I added a feature request in YouTrack: https://youtrack.jetbrains.com/issue/PY-23190

[davidism]

While we now document this in Flask, I encourage anyone who comes across this to go vote for the linked PyCharm issue. You'll need to log in, then there's a small thumbs up icon at the bottom of the right-hand sidebar (the star is not the same as the vote button).