Decrease default Synapse logging level

Also discussed previously in #213 (Github Pull Request).

shared-secret-auth and rest-auth logging is still at `INFO`
intentionally, as user login events seem more important to keep.
Those modules typically don't spam as much.

CHANGELOG.md

@@ -1,3 +1,18 @@
+# 2019-08-03
+
+## Synapse logging level has been reduced to WARNING
+
+After [some discussion in our support room](https://matrix.to/#/!PukFFdIcHgtaaHZflT:devture.com/$156476852524179TBeKy:matrix.org?via=devture.com&via=matrix.org&via=librem.one), we've decided to change the default logging level for Synapse from `INFO` to `WARNING`.
+
+This greatly reduces the number of log messages that are being logged, leading to:
+
+- much less disk space dedicated to Synapse and thus, logs kept for longer
+- easier to find some important `WARNING`, `ERROR` and `CRITICAL` messages, as they're not longer buried in thousands of non-important `INFO` messages
+
+If you'd like to track down an issue, you [can always increase the logging level as described here](./docs/maintenance-and-troubleshooting.md#increasing-synapse-logging).
+
+
+
 # 2019-07-08
 
 ## Synapse Maintenance docs and synapse-janitor support are available