Make a note to leave a summary when one is bumping the schema version (#10621)

I found this easy to miss (and evidently, it looks like it was missed for schema version 62).

changelog.d/10621.misc

+Add a comment asking developers to leave a reason when bumping the database schema version.
\ No newline at end of file

synapse/storage/schema/__init__.py

@@ -12,6 +12,8 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
+# When updating these values, please leave a short summary of the changes below.
+
 SCHEMA_VERSION = 63
 """Represents the expectations made by the codebase about the database schema