Added some user documentation.

README.md

@@ -1,2 +1,15 @@
-# asl_cards
-A UI to view cards in the "ASL Cards" PDF's.
+# ASL Cards
+
+If you're a fan of [ASL](https://en.wikipedia.org/wiki/Advanced_Squad_Leader), then Chris Edward's [ASL Cards](http://aslcards.com) are a useful play tool. He provides PDF's with data cards for every AFV and Gun in the system, which are handy to reference as you play a scenario.
+
+However, unless you print them out, it's difficult to quickly switch between the ones you want, so I wrote this program that lets you pick out the ones you want, with a tabbed viewer that lets you quickly flip between them.
+
+### Analyzing the PDF files
+<img src="https://github.com/pacman-ghost/asl_cards/doc/analyze-files.png" style="width:300px;float:left;margin-right:10px;">The first time you run the program, it needs to analyze the PDF files, to extract each data card (this only needs to be done once).
+
+Simply point to the directory where the files live, and click Analyze. This process can take some time to run, ~5-10 minutes at the lowest resolution, ~1 hour at the highest.
+
+### Selecting cards
+<img src="https://github.com/pacman-ghost/asl_cards/doc/add-card.png" style="width:300px;float:left;margin-right:10px;">Once the files have been analyzed, you can pick out the cards you want.
+<br clear="all">
+<img src="https://github.com/pacman-ghost/asl_cards/doc/view-cards.png" style="width:300px;float:left;margin-right:10px;">The cards are now easily accessible as you play the scenario.