Add description to README

Author: Tearth

README.md

@@ -1,2 +1,22 @@
 # PGN2ABK
-Fast generator of chess openings book with support for Arena's book format (ABK).
+This is the simple tool to generate ABK (Arena's book format) files using a collection of games saved in the PGN file. There are several sources of PGN files, my favorite one is https://database.lichess.org/. It takes about 30 minutes to parse 190 GB file on normal HDD.
+
+# Usage
+
+```
+-i, --input             Required. PGN input file to process.
+-o, --output            Required. ABK output file.
+-p, --plies             (Default: 2147483647) Maximal number of plies to parse.
+-e, --elo               Minimal average ELO of players to parse the game.
+-t, --time              Minimal initial game time to parse the game.
+-m, --multithreading    Enable support for multithreading.
+--help                  Display this help screen.
+--version               Display version information.
+```
+
+# Example
+
+Input: `-i D:\DB\lichess_db_standard_rated_2013-01.pgn -o C:\Users\Pawel\Desktop\generated.abk -p 20 -m`
+
+Output:
+![Output](https://i.imgur.com/J4FcrwN.png)