Extend README

Cleanup, add license badge, move license file to root dir, document database use with wpp

README.md

-WowPacketParser
-========
+WowPacketParser (WPP)
+=====================
 
+[![GitHub license](https://img.shields.io/github/license/TrinityCore/WowPacketParser.svg?style=flat-square)](https://github.com/TrinityCore/WowPacketParser/blob/WowPacketParser/COPYING)
 [![Coverity Scan Build Status](https://img.shields.io/coverity/scan/2618.svg?style=flat-square)](https://scan.coverity.com/projects/2618)
-
 [![Build Status TravisCI](https://img.shields.io/travis/TrinityCore/WowPacketParser.svg?style=flat-square)](https://travis-ci.org/TrinityCore/WowPacketParser)
 [![Build status AppVeyor](https://img.shields.io/appveyor/ci/DDuarte/wowpacketparser-191.svg?style=flat-square)](https://ci.appveyor.com/project/DDuarte/wowpacketparser-191)
 
 Usage
 -----
 
-* Compile WowPacketParser using Debug or Release on Visual Studio 2015 or higher.
-* Edit "WowPacketParser.exe.config" to fit your needs.
-* Drag files to the executable.
+* Compile WowPacketParser using Visual Studio 2015 or higher (Windows) or Mono 4.0 or higher (Linux/OSX).
+  Alternatively you can download compiled binaries from the links [below](#nightly-builds).
+* Edit `WowPacketParser.exe.config` to fit your needs.
+* Drag one or more files (.pkt or .bin) to `WowPacketParser.exe`.
+* Command line usage: `WowPacketParser.exe [--ConfigFile path --Option1 value1 ...] filetoparse1 ...`
+
+##### Databases
+
+Optionally, WPP can connect to two kinds of MySQL databases: `world` from [TrinityCore](https://github.com/TrinityCore/TrinityCore)
+and its own database, `WPP`. This can be enabled by setting `<add key="DBEnabled" value="true" />`
+in the `.config` file.
+
+The `world` database is used when creating SQL files after parsing to produce the mimimum number of
+changes needed to update the database. For example, if only the faction of a creature that appears
+in the sniff needs to be updated, the produced SQL files will contain an UPDATE query, instead of
+a full INSERT to the table `creature_template`. WPP does not modify this database directly, all the
+tentative changes will be written to the output SQL files.
 
+The `WPP` database is used to feed additional data that WPP may use while parsing. For example, in
+the output text files, the spell name can be displayed next to spell ids:
+
+> ServerToClient: SMSG_SPELL_START (0x2BB8) Length: 96 ConnIdx: 0 Time: 01/01/2016 00:22:33.235 Number: 701  
+> (Cast) CasterGUID: Full: 0x03691F00000000000000000000000001 Player/0 R3558/S0 Map: 0 Low: 1  
+> (Cast) SpellID: **2479 (Honorless Target)**  
+> (Cast) CastFlags: 15
+
+This is available for a lot of other named entities (achievements, creatures, quests, etc.). The SQL
+files required for this database is in the `SQL` directory. `create_WPP.sql` creates the database
+and `wpp_data_objectnames.sql` has some data to fill the database.
 
 Nightly Builds
 --------------
 
-- [Debug 32/64 Bits](https://ci.appveyor.com/api/projects/DDuarte/wowpacketparser-191/artifacts/WowPacketParser/WPP.zip?job=Configuration:%20Debug)
-- [Release 32/64 Bits](https://ci.appveyor.com/api/projects/DDuarte/wowpacketparser-191/artifacts/WowPacketParser/WPP.zip?job=Configuration:%20Release)
+##### .NET 4.5 (AnyCPU) by AppVeyor
+
+- [Debug](https://ci.appveyor.com/api/projects/DDuarte/wowpacketparser-191/artifacts/WowPacketParser/WPP.zip?job=Configuration:%20Debug)
+- [Release](https://ci.appveyor.com/api/projects/DDuarte/wowpacketparser-191/artifacts/WowPacketParser/WPP.zip?job=Configuration:%20Release)