Update README with installation instructions

Also changes around some styling and adds a License section (though deliberately doesn't add a `LICENSE` file - GitHub should not be recognizing this project as MIT Licensed as the vast majority of the bytes are not subject to that license). Without any mention of a license potential contributors would be allowed to fork and view under the GitHub ToS, but technically making changes to the code they've forked is not covered.

A future commit should add a `CREDITS.md` file linked to from a `Credits` section in this README detailing those who have contributed their work to this repository.

README.md

@@ -1,29 +1,61 @@
+# Pokémon Sprites
 
-The Smogon/Pokemon-Showdown sprite repository
+> The Smogon / Pokémon Showdown! sprite repository.
 
-# Installation
+## Installation
 
-- Tup build system. Instructions for Windows/Mac/Linux can be found [here](http://gittup.org/tup/).
-- GraphicsMagick
-- AdvPng
+This project depends on
 
-# Building
+- [tup](http://gittup.org/tup/)
+- [GraphicsMagick](http://www.graphicsmagick.org/)
+- [AdvPng](http://www.advancemame.it/doc-advpng.html)
 
-Run `tup`.
+### Windows
 
-# Filename scheme
+Windows binaries of these dependencies can be found on the download pages of the sites listed above.
 
-Pokemon sprite filenames are in a 1-to-1 correspondence with the Pokemon's name, for ease of processing. Filenames may be directly substituted in shell commands without escaping.
+### Linux
+
+_**TODO:** include installation instructions for `tup` given the Ubuntu PPA is defunct_
+
+```
+$ sudo apt install graphicsmagick advancecomp
+```
+
+### macOS
+
+Using [`brew`](https://brew.sh/) on  a macOS:
+
+```
+$ brew cask install osxfuse
+$ brew install tup graphicsmagick advancecomp
+```
+
+## Building
+
+```
+$ tup
+```
+
+## Filename Scheme
+
+Pokemon sprite filenames are in a 1-to-1 correspondence with the Pokemon's name, for ease of processing. Filenames may be directly substituted in shell commands without escaping. This naming scheme means that some of the filenames in `src/` are a little awkward looking to humans, but it means that no additional data beyond what is encoded in the filesystem is required to determine the correct name for any given Pokémon. 
 
 - Characters outside `[0-9a-zA-Z-.]` will be escaped using `_` and four hex characters, similar to JavaScript Unicode escapes. This conforms to the POSIX portable filename character set. (example: `Flabébé` -> `Flabe_0301be_0301`)
 
-The following JS function may be useful:
-```javascript
-function encode(s) {
-    return s.replace(/[^0-9a-zA-Z-.]/g, c => '_' + c.charCodeAt(0).toString(16).padStart(4, '0'));
-}
-```
+    The following JS function may be useful:
+     ```javascript
+    function encode(s) {
+        return s.replace(/[^0-9a-zA-Z-.]/g, c => '_' + c.charCodeAt(0).toString(16).padStart(4, '0'));
+    }
+    ```
 
 - Formes are separated with two dashes from their base. (example: `Necrozma--Dawn-Wings`)
 
 - Cosmetic female formes are `--Female` instead of `--F`, so that you may distinguish it from Unown.
+
+## License
+
+All code in this repository is licensed under the [MIT License](https://opensource.org/licenses/MIT).
+
+The sprites themselves are property of Nintendo / Game Freak / The Pokémon Company, though Black & White sprites for Pokémon from later generations were created by artists in the community. The license for these community-created sprites is still being determined and may change in the future, but in the meantime please talk to use first before using them.