Update README.md with the parallelization bits

This patch updates the README.md to mention the `CONCURRENCY`
environmental variable that can be used to control the new
parallelization functionality.

Before parallelizing the work the timing of a must-gather in my single
node CRC deployment took:

- 13m35s without SOS reports
- 15m49s with SOS reports

After this parallelization effort it takes

- 7m9s without SOS report
- 7m40s with SOS report

README.md

@@ -38,6 +38,9 @@ oc adm must-gather --image=quay.io/openstack-k8s-operators/openstack-must-gather
 
 This is the list of available environmental variables:
 
+- `CONCURRENCY`: Must gather runs many operations, so to speed things up we run
+  them in parallel with a concurrency of 5 by default. Users can change this
+  environmental variable to adjust to its needs.
 - `SOS_SERVICES`: Comma separated list of services to gather SOS reports from.
   Empty string skips sos report gathering. Eg: `cinder,glance`. Defaults to all
   of them.