Add README

shell-completion/README.md

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+Shell completion for CLI
+========================
+
+This code generates shell completion scripts for hledger's command line
+interface.
+Shell completion is usually triggered by pressing the tab key once or twice
+after typing the command `hledger `.
+(The exact behavior may differ in shells other than Bash.)
+
+Currently, only Bash is supported but Zsh or Fish can be added.
+
+[Demonstration video](https://asciinema.org/a/PdV2PzIU9oDQg1K5FjAX9n3vL)
+
+The completions can handle hledger's CLI:
+
+- commands and generic options
+- command-specific options
+- account names (but not yet for files named by --file)
+- filenames for options that take a filename as argument
+
+Installation
+------------
+
+First, generate the completion script for Bash:
+
+```
+# change into this folder:
+cd shell-completion/
+make
+```
+
+Hint: GNU make, GNU m4, and GNU parallel must be installed to call `make`.
+The first two usually are.
+
+Then, the generated completion script must be installed. TBD.
+For now, you can use this two commands:
+
+```
+cp hledger-completion.bash ~/.hledger-completion.bash
+echo 'source ~/.hledger-completion.bash' >> ~/.bashrc
+```
+
+After that, you have to start a new Bash, e.g. by typing `bash` on the current
+shell.
+
+Now, try it by typing `hledger` (with a space after the command) and press the
+tab key twice. Then you can type a part of one of the suggestions and press tab
+again to complete it.
+
+Background
+----------
+
+The Bash completion script is generated (GNU make) by parsing output of hledger,
+hledger -h, and hledger <cmd> -h. The script also uses hledger accounts for
+account name completion. I propose that the Makefile is not run at every built
+but rather manually when the CLI changes.