Add note in template about asking for preliminary feedback

Allow people to post the first half of a spec to get feedback on the
problem statement and outline of the solution. If we don't think the
problem should be fixed in nova, then no point in making the author fill
out all the details about the fix.

Change-Id: Ic0aa4d870d8ab8c8c8b13246dba991a733ac21bd
Merge "Add note in template about asking for preliminary feedback"