Add example using GNU parallel to run tasks in concurrently (#1915)

README.md

@@ -3061,6 +3061,19 @@ watchexec just foo
 See `watchexec --help` for more info, including how to specify which files
 should be watched for changes.
 
+### Running tasks in parallel
+
+GNU parallel can be used to run tasks concurrently:
+
+```just
+parallel:
+  #!/usr/bin/env -S parallel --shebang --ungroup --jobs {{ num_cpus() }}
+  echo task 1 start; sleep 3; echo task 1 done
+  echo task 2 start; sleep 3; echo task 2 done
+  echo task 3 start; sleep 3; echo task 3 done
+  echo task 4 start; sleep 3; echo task 4 done
+```
+
 ### Shell Alias
 
 For lightning-fast command running, put `alias j=just` in your shell's