Add migrations help

docs/bridge-migrations.md

+# 1.0 Migration (from 0.5.1 or lower)
+
+If you have been linked here, there is an issue with your config on your bridge.
+
+Please follow the following steps:
+
+1. If you have just created a new install OR were previously running 0.5.X,
+   please remove `roomDataStore` and `userDataStore` from your config file.
+2. If this is a existing install but you have not run 0.5.X (0.4.X or lower),
+   please downgrade to 0.5.X to migrate your database across and then run
+   this version of the bridge again.