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).

2 files changed, 3 insertions, 0 deletions

diff --git a/changelog.d/10621.misc b/changelog.d/10621.misc
new file mode 100644

index 0000000000..b8de2e1911
--- /dev/null
+++ b/changelog.d/10621.misc
@@ -0,0 +1 @@
+Add a comment asking developers to leave a reason when bumping the database schema version.
\ No newline at end of file
diff --git a/synapse/storage/schema/__init__.py b/synapse/storage/schema/__init__.py

index a5bc0ee8a5..af9cc69949 100644
--- a/synapse/storage/schema/__init__.py
+++ b/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