Add usage documentation

docs/usage.md

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+# Usage
+    $ mdp --help
+    usage: mdp [-h] [-v] [--update-csl]
+    
+    optional arguments:
+      -h, --help     show this help message and exit
+      -v, --verbose  Set verbosity level (repeat argument)
+      --update-csl   Update CSL files
+
+## Running the application
+Assuming you're in a directory with a valid `mdp.yml` file, simply run `mdp` to build your files to the output directory!
+
+## Downloading CSL Files
+In order to use referencing properly, `csl` files need to be downloaded. These tell `md-pdf` what style of referencing you're using, and how to output the bibliography.
+
+To download these, simply run:
+
+    mdp --update-csl
+
+This will download the `csl` files into the install directory of the application. It only needs to be run once, however can be run whenever to update the definitions from [here](https://github.com/citation-style-language/styles/).
+
+## Verbosity
+By default, when building, the application doesn't output anything to the console. It simply runs, and terminates when it's finished. Obviously errors are still reported.
+
+To increase the verbosity level, simply add the `-v` argument when running the application. This can be chained a maximum of 3 times (`-vvv`), subsequent repetition will be ignored.