Create release notes template

[rjyounes]

Something along the lines of:

Release X.x.x
=====
 
Major changes
-----
 
### Name
   - Description:
   - Issue(s):
   - Impact:
   - Migration plan:

Minor changes
-----
### Name
   - Description:
   - Issue(s):
   - Impact:
 
Patch changes
----- 
### Name
   - Description:
   - Issue(s):

Each PR should be accompanied by an entry in the release notes. If the developer has not made this entry, the PR should be rejected. Documentation should be updated to this effect.

[rjyounes]

Rebecca: Submit PR.

[rjyounes]

Added an additional line:

Release X.x.x
-----
 
### Major changes

#### Name
   - Description:
   - Motivation/Rationale:
   - Impact:
   - Migration plan:
   - Issue(s):

### Minor changes

#### Name
   - Description:
   - Motivation/Rationale:
   - Impact:
   - Issue(s):
 
### Patch changes

#### Name
   - Description:
   - Motivation/Rationale:
   - Issue(s):

Motivation/Rationale for patch changes may be as simple as "Bug fix."