docs: promote multithreading section

I think now it's a bit odd it's in Usage - there are are top level "appendix-y"
sections that it's more similar to.

Author: michalc

README.md

@@ -218,7 +218,8 @@ with \
         print(row)
 ```
 
-### Multithreading
+
+## Multithreading
 
 It is safe for multiple threads to call the same `query` function. Under the hood, each use of `query` uses a separate SQLite "connection" to the database combined with the `SQLITE_OPEN_NOMUTEX` flag, which makes this safe while not locking unnecessarily.