update README with instructions for sync command (#66)

Author: SirNeuman

README.md

@@ -100,6 +100,10 @@ If you have a large organization, you may wish to tag ownership of splits to a s
 3. If the test track client is able to find this file, it will require an `--owner` flag be set when creating new splits and experiements.
 4. This data will be passed to the test track server where it can be recorded on the split records
 
+### Syncing split assignments
+
+If you want to ensure that your local split assignments are in sync with your remote (production) assignments, you can run `TESTTRACK_CLI_URL=<base_url> testtrack sync` (e.g. `TESTTRACK_CLI_URL=https://tt.example.com testtrack sync`) from your project directory to pull the assignments from your remote server into your local `schema.yml` file.
+
 ## How to Contribute
 
 We would love for you to contribute! Anything that benefits the majority of TestTrack users—from a documentation fix to an entirely new feature—is encouraged.