Add information from #2 to README about being race-safe

Signed-off-by: Olivier Wilkinson (reivilibre) <olivier@librepush.net>

README.md

@@ -10,6 +10,14 @@ given file that would change the tables to match the calculated state. (Note
 that if `-t` is given then each change to a particular state group is wrapped
 in a transaction).
 
+The SQL generated by the `-o` option is safe to apply against the database with
+Synapse running. This is because the `state_groups` and `state_groups_state`
+tables are append-only: once written to the database, they are never modified.
+There is therefore no danger of a modification racing against a running synapse.
+Further, this script makes its changes within atomic transactions, and each
+transaction should not affect the results from any of the queries that synapse
+performs.
+
 The tool will also ensure that the generated state deltas do give the same state
 as the existing state deltas.