Add some clarifications to README.md in the database schema directory. (#6615)

Signed-off-by: Ruben Barkow-Kuder <github@r.z11.de>

changelog.d/6615.misc

+Add some clarifications to `README.md` in the database schema directory.

synapse/storage/data_stores/main/schema/full_schemas/README.md

-# Building full schema dumps
+# Synapse Database Schemas
 
-These schemas need to be made from a database that has had all background updates run.
+These schemas are used as a basis to create brand new Synapse databases, on both
+SQLite3 and Postgres.
 
-To do so, use `scripts-dev/make_full_schema.sh`. This will produce
-`full.sql.postgres ` and `full.sql.sqlite` files.
+## Building full schema dumps
+
+If you want to recreate these schemas, they need to be made from a database that
+has had all background updates run.
+
+To do so, use `scripts-dev/make_full_schema.sh`. This will produce new
+`full.sql.postgres ` and `full.sql.sqlite` files. 
 
 Ensure postgres is installed and your user has the ability to run bash commands
-such as `createdb`.
+such as `createdb`, then call
+
+    ./scripts-dev/make_full_schema.sh -p postgres_username -o output_dir/
 
-```
-./scripts-dev/make_full_schema.sh -p postgres_username -o output_dir/
-```
+There are currently two folders with full-schema snapshots. `16` is a snapshot
+from 2015, for historical reference. The other contains the most recent full
+schema snapshot.