Add installation and usage instructions to README

README.md

@@ -2,6 +2,71 @@
 
 *A code formatter with rules set in stone.*
 
+## Installation
+
+```julia
+using Pkg
+Pkg.add("Runic")
+```
+
+## Usage
+
+The main interface to Runic is the command line interface (CLI) through the `main` function
+invoked with the `-m` flag. See the output of `julia -m Runic --help` for details:
+
+> [!NOTE]
+> The `-m` command line flag is only available in Julia 1.12 and later. In earlier versions
+> you have to invoke the `main` function explicitly, for example:
+> ```sh
+> julia -e 'using Runic; exit(Runic.main(ARGS))' -- <args>
+> ```
+
+```sh
+$ julia-master -m Runic --help
+NAME
+       Runic.main - format Julia source code
+
+SYNOPSIS
+       julia -m Runic [<options>] <path>...
+
+DESCRIPTION
+       `Runic.main` (typically invoked as `julia -m Runic`) formats Julia source
+       code using the Runic.jl formatter.
+
+OPTIONS
+       <path>...
+           Input path(s) (files and/or directories) to process. For directories,
+           all files (recursively) with the '*.jl' suffix are used as input files.
+           If path is `-` input is read from stdin.
+
+       -c, --check
+           Do not write output and exit with a non-zero code if the input is not
+           formatted correctly.
+
+       -d, --diff
+           Print the diff between the input and formatted output to stderr.
+           Requires `git` or `diff` to be installed.
+
+       --fail-fast
+           Exit immediately after the first error. Only applicable when formatting
+           multiple files in the same invocation.
+
+       --help
+           Print this message.
+
+       -i, --inplace
+           Edit files in place. This option is required when passing multiple input
+           paths.
+
+       -o, --output <file>
+           Output file to write formatted code to. If the specified file is `-`
+           output is written to stdout. This option can not be used together with
+           multiple input paths.
+```
+
+In addition to the CLI there is also the two function `Runic.format_file` and
+`Runic.format_string`. See their respective docstrings for details.
+
 ## Formatting specification
 
 This is a list of the rules and formatting transformations performed by Runic: