Update README about disabling ratelimits

To help alleviate #73

redash_toolbelt/docs/redash-migrate/README.md

@@ -168,6 +168,11 @@ Commands are idempotent. The same query, dashboard, visualization etc. will not
 If you run `redash-migrate queries` repeatedly, the command will only attempt to import queries that
 were not successfully migrated during a previous execution.
 
+**Note**: due to the volume of network requests it creates, redash-migrate may trip the rate limit
+on your destination Redash instance. We recommend you disable rate limiting at the destination 
+for the duration of your migration. On Redash V10 this can be disabled by setting an environment
+variable named `REDASH_RATELIMIT_ENABLED=false`.
+
 ## FAQ
 
 ### Who should use redash-migrate?
@@ -243,4 +248,4 @@ redash-migrate is released into the public domain by the copyright holders.
 
 This README file was originally written by Jesse Whitehouse and is likewise released to the public
 domain. This README file is based on the work of Daniel Bolton for youtube-dl with deep gratitude
-for that effort.
+for that effort.