shell_command_after: fix doc and add real life example

[simahawk]

Current result w/ README example:

expected <block end>, but found '<block mapping start>'
  in "<string>", line 43, column 5:
        shell_command_after:
        ^

This is fixed and I added an example on how to apply a patch (just tested w/ latest odoo sec patch).

[coveralls]

Coverage remained the same at 81.953% when pulling 343463a on simahawk:master into fe0a1d3 on acsone:master.

[sbidoul]

Nice improvement, thanks!