Add uninstallation information to README.md

Brief instructions about the `--uninstall` switch, directly after
installation information. Also fix a typo later in the file.

Change-Id: Idddf5e59172950aef3afac6698c1c7750bfc2465
Reviewed-on: https://gerrit.causes.com/23255
Tested-by: jenkins <jenkins@causes.com>
Reviewed-by: Aiden Scandella <aiden@causes.com>