[Fuchsia] update build documentation

Add explicit instructions on how to add a userid to
the `kvm` group.

Bug: None
Change-Id: Iea1432c2213a9b8f1b5875c91727963e83cc747a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2097158
Commit-Queue: Scott Graham <scottmg@chromium.org>
Reviewed-by: Scott Graham <scottmg@chromium.org>
Cr-Commit-Position: refs/heads/master@{#749205}

docs/fuchsia_build_instructions.md

@@ -190,6 +190,11 @@ $ sudo apt-get install qemu-system-common
 ```
 3. Add users to the "kvm" group, and have them login again, to pick-up the new
 group.
+```shell
+$ sudo adduser <user> kvm
+$ exit
+[log in again]
+```
 
 ### Running test suites