Add README.md for migrate_gid_to_uid tool.

tools/migrate_gid_to_uid/README.md

@@ -0,0 +1,44 @@
+## migrate_gid_to_uid
+
+This tool will migrate all device data with a _groupId field and add a _userId field containing the associated user id. It should be executed immediately after upgrading to any version of the `platform` *after* `v0.1.0`.
+
+To execute this tool against a local development setup:
+
+1. Prepare the environment and build the executables. For more information, please see the main README.md.
+
+  ```
+  source .env
+  make build
+  ```
+
+1. Run the tool against the Mongo database running on `localhost`.
+
+  ```
+  _bin/tools/migrate_gid_to_uid/migrate_gid_to_uid -a localhost
+  ```
+
+  There may be a number of warnings if the local development setup has partially created accounts and/or very old device data.
+
+  There should be a final log message along the lines of:
+
+  ```
+  {"file":"tools/migrate_gid_to_uid/migrate_gid_to_uid.go","level":"info","line":455,"msg":"Migrated 13538 device data for 4 groups","pid":23991,"process":"migrate_gid_to_uid","time":"2016-09-06T13:45:58-07:00","version":"0.1.3+88d5568"}
+  ```
+
+  Note this log message reports `Migrated 13538 device data for 4 groups`. This means that the device data for 4 users was properly migrated.
+
+1. Run the tool again, just to ensure all data has been migrated.
+
+  ```
+  _bin/tools/migrate_gid_to_uid/migrate_gid_to_uid -a localhost
+  ```
+
+  Again, there may be a number of warnings if the local development setup has partially created accounts and/or very old data.
+
+  There should be a final log message along the lines of:
+
+  ```
+  {"file":"tools/migrate_gid_to_uid/migrate_gid_to_uid.go","level":"info","line":455,"msg":"Migrated 0 device data for 0 groups","pid":23991,"process":"migrate_gid_to_uid","time":"2016-09-06T13:45:58-07:00","version":"0.1.3+88d5568"}
+  ```
+
+  Note that this log message reports `Migrated 0 device data for 0 groups`. This means that all device data was properly migrated during the previous step. This is the expected final response.