Add NOTES.txt check

[trgeiger]

Checks for NOTES.txt in the chart's templates directory. Includes tests.

Closes #428

Right now it's just another mandatory check since we don't have any logic for optional checks. I'm not sure how everyone feels about optional vs. mandatory.

[komish]

@trgeiger thanks for this - just two things

• NOTES.txt is intended to convey information to the end-user, but some charts may not have information to convey. If we're going to add this check, it probably needs to be optional across all profiles.
• Any references to "Notes" should specify "NOTES.txt", so that our expectations are clear.

[komish]

/lgtm

[komish]

Thanks @trgeiger