Add details to the README

Add details of what Librarian is to the README file.

Signed-off-by: Jason Rogena <[email protected]>

README.md

@@ -1,3 +1,63 @@
 ## Librarian
 
 [![Build Status](https://github.com/jasonrogena/librarian/workflows/CI/badge.svg)](https://github.com/jasonrogena/librarian/actions?query=workflow%3ACI)[![codecov](https://codecov.io/gh/jasonrogena/librarian/branch/main/graph/badge.svg?token=O3PNGORLW8)](https://codecov.io/gh/jasonrogena/librarian)
+
+Librarian searches for files in your filesystem and runs the configured commands against discovered files.
+
+To run Librarian once, where it exits after searching through all the libraries, run:
+
+```sh
+librarian single-shot path/to/config.toml
+```
+
+To make Librarian continually watch for when files in the libraries change (are created or updated), run:
+
+```sh
+librarian watch path/to/config.toml
+```
+
+### Configuration
+
+An example configuration file can be found [here](./tests/configs/good.toml).
+
+#### Libraries
+
+In the Librarian configuration file, define one or more "libraries" of files. A library is a set of files that match defined search filters. Supported search filters are:
+
+- A required list of parent directories the file can be in
+- An optional list of regexes the file's MIME type should match
+
+For each of the defined libraries, provide a [Jinja2](https://jinja.palletsprojects.com/en/2.10.x/templates/) template of the command that should run when a file is found. The following variables are available to the template:
+
+- `{{ file_path }}`: The path to the file that was found
+- `{{ mime_type }}`: The MIME type for the file that was found
+
+The following configuration snippet defines a music library which watches for files inside the Downloads and /tmp directories that have MIME types matching the `audio/.+` regex (e.g. `audio/flac` and `audio/ogg`). When an audio file is found, it is moved to the Music directory:
+
+```toml
+[libraries.music]
+command = """
+mv "{{ file_path }}" /home/jrogena/Music/
+"""
+
+  [libraries.music.filter]
+  directories = [ "/home/jrogena/Downloads", "/tmp" ]
+  mime_type_regexes = [ "audio/.+" ]
+```
+
+#### Filesystem Watching
+
+The following configurations, related to filesystem watching, are available:
+
+ - `min_command_exec_freq`: Optional. The minimum frequency (in seconds) between running the configured command against a file. Useful in situations where a file is updated frequently but you don't want Librarian to run against the file as frequently as it is updated.
+
+ The following snippet is an example filesystem watching configuration:
+
+```toml
+[fs_watch]
+min_command_exec_freq = 60
+```
+
+### License
+
+[MIT](./LICENSE)