docs: add shell completion information

docs/source/index.rst

@@ -13,3 +13,4 @@ Welcome to checkcert's documentation!
    introduction
    install
    running
+   shell_completion

docs/source/shell_completion.rst

@@ -0,0 +1,60 @@
+Shell Completion
+================
+
+In order to use shell completion, checkcert must first be installed via ``pip install checkcert``.  That generates the `checkcert` entry point that ultimately is used for the command completion.  These instructions will not work if using a git cloned version.
+
+Completion scripts are provided in the source's "completion" directory.  You may copy those to source in your shell's rc files.
+
+These steps are basically copied from Click's Documentation, since that is what is generating the completions.  Check `Click's completion documentation <https://click.palletsprojects.com/en/8.0.x/shell-completion/>`_ for more details.
+
+Obtaining from github
+---------------------
+
+Download the needed completion script from github.  Using curl, you could download the zsh completion with
+
+``curl -LO https://raw.githubusercontent.com/kellya/checkcert/main/completions/checkcert-complete.zsh``
+
+Then just append ``source /path/to/checkcert-complete.zsh`` to your .zshrc.
+
+Bash will be pretty much the same, just use:
+
+``curl -LO https://raw.githubusercontent.com/kellya/checkcert/main/completions/checkcert-complete.sh`` instead
+
+
+Generating Completion script
+----------------------------
+
+If you do not have access to the files from github, but have checkcert installed, you may generate the completion scripts for inclusion in your shell rc file.
+
+zsh
+^^^
+
+``_CHECKCERT_COMPLETE=zsh_source checkcert > ~/.checkcert-complete.zsh``
+
+Then in .zshrc, add a ``source ~/.checkcert-complete.zsh``
+
+.. note:: There are various plugin directories that could be used to automatically install a completion.  There are many options, so this doc just highlights a way that will work.
+
+bash
+^^^^
+
+The bash method is pretty much the same as zsh.
+
+``_CHECKCERT_COMPLETE=bash_source checkcert > ~/.checkcert-complete.bash``
+
+Generating Completion via eval
+------------------------------
+
+Instead of generating a script to execute, you may use eval to generate the completions.  This is a little quicker to implement; however there is a speed trade-off as the shell has to run this each time.
+
+zsh
+^^^
+
+Execute the following: ``eval "$(_CHECKCERT_COMPLETE=zsh_source checkcert)"``.  You may put this in your ~/.zshrc to persist the setting.
+
+bash
+^^^^
+
+Execute the following: ``eval "$(_CHECKCERT_COMPLETE=bash_source checkcert)"``
+
+