Document about `php-tokenizer` requirement

[mikaelkundert]

While commiting, I got this error:

$ git commit -a
GrumPHP detected a pre-commit command.
GrumPHP is sniffing your code!
Running task 1/7: EcsTask... ✔
Running task 2/7: PhpcsTask... ✘
Aborted ...
ERROR: PHP_CodeSniffer requires the tokenizer, xmlwriter and SimpleXML extensions to be enabled. Please enable xmlwriter and SimpleXML.

To skip commit checks, add -n or --no-verify flag to commit command

Apparently I have to install php-tokenizer package on my host machine to be able to use this.