Update release instructions

Author: bentsherman

README.md

@@ -55,8 +55,9 @@ A separate branch is maintained for each stable release, starting with `STABLE-2
 
 To make a new release of the language server:
 
-1. Build the language server locally.
-2. Create a new GitHub release with the language server JAR and a list of notable changes.
+1. Checkout the appropriate `STABLE-*` branch for Nextflow.
+2. Build the language server locally.
+3. Create a new GitHub release with the language server JAR and a list of notable changes.
 
 ## Troubleshooting