Document command for Homebrew plugin install

[ivy]

This is a small change to the Homebrew install docs to recommend a one-liner instead of the manual approach:

$(brew --prefix certbot)/libexec/bin/pip install certbot-s3front

[ivy]

@dlapiduz Just a friendly reminder to have a look at this PR when you have some time. Let me know if you need more info or if something needs to change. 😄

[dlapiduz]

Thanks for the PR. I added a quick comment, once that is clear we can merge it.

[ivy]

@dlapiduz Did you have any other questions? Is there anything I can do to boost your confidence that these docs are correct?

---

First, the resolved path of brew --prefix:

$ ls -Ahl $(brew --prefix certbot)
lrwxr-xr-x  1 ivy  admin    24B Aug  7 13:25 /usr/local/opt/certbot@ -> ../Cellar/certbot/0.26.1

Then, a peek into the directory itself:

$ tree -L 3 $(brew --prefix certbot)
 /usr/local/opt/certbot
├── AUTHORS.md
├── CHANGELOG.md
├── CHANGES.rst
├── INSTALL_RECEIPT.json
├── LICENSE.txt
├── README.rst
├── bin
│   └── certbot -> ../libexec/bin/certbot
├── libexec
│   ├── bin
│   │   ├── __pycache__
│   │   ├── activate
│   │   ├── activate.csh
│   │   ├── activate.fish
│   │   ├── activate_this.py
│   │   ├── certbot
│   │   ├── chardetect
│   │   ├── easy_install
│   │   ├── easy_install-3.7
│   │   ├── futurize
│   │   ├── jp.py
│   │   ├── jws
│   │   ├── pasteurize
│   │   ├── pbr
│   │   ├── pip
│   │   ├── pip3
│   │   ├── pip3.7
│   │   ├── python -> python3.7
│   │   ├── python-config
│   │   ├── python3 -> python3.7
│   │   ├── python3.7
│   │   ├── rst2html.py
│   │   ├── rst2html4.py
│   │   ├── rst2html5.py
│   │   ├── rst2latex.py
│   │   ├── rst2man.py
│   │   ├── rst2odt.py
│   │   ├── rst2odt_prepstyles.py
│   │   ├── rst2pseudoxml.py
│   │   ├── rst2s5.py
│   │   ├── rst2xetex.py
│   │   ├── rst2xml.py
│   │   ├── rstpep2html.py
│   │   └── wheel
│   ├── include
│   │   └── python3.7m -> ../../../../../opt/python/Frameworks/Python.framework/Versions/3.7/include/python3.7m
│   ├── lib
│   │   └── python3.7
│   └── pip-selfcheck.json
└── share
    └── certbot
        └── examples

11 directories, 41 files

The Homebrew certbot formula seems to vendor its own Python, as you can see by the resulting binaries with only a couple symlinks. I'm not too familiar with how virtualenv works but I'm pretty sure the plugin only needs a reinstall when an update to certbot is installed (since /usr/local/opt/certbot will then point to a new path: ../Cellar/certbot/x.x.x).