Recommend installing with `pipx`

[kalekundert]

This PR changes the documentation to recommend installing with pipx rather than pip. There are two reasons why I think this is a useful change:

• pipx installs packages into their own virtual environments, so their dependencies are completely isolated from the rest of the system. My understanding is that this is currently considered the best practice when it comes to installing packages that are meant to be used via the command-line (as opposed to those that are meant to be used as libraries), and I think gitlint falls squarely in that category.
• The pipx install command isn't trivial, in that it requires the unusual --include-deps flag. The reason is that the gitlint package is a sort-of dummy package that pins dependency versions but doesn't directly provide the executable. pipx notices this and complains about it, unless the above flag is given.

If you don't want to change the recommendation from pip to pipx, feel free to close this PR. I just made it because it took me a little bit to figure out what was going on, and I thought it might save some other people the trouble.

[coveralls]

coverage: 99.633%. remained the same when pulling 02d42aa01d56f255c7cc9cb7ae07770d9d99cc3c on kalekundert:docs-pipx into 4d9119760056492eabc201bfad5de2f9e660b85f on jorisroovers:main.