Moved wiki docs into main project.

Why have them separate? Github allows reading docs right in the main tree.

docs/SchemaMigration.md

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+_Overview of Schema and Data Migration._
+
+## Schema Migration
+
+Depot makes simple schema migrations extremely simple and complex schema migrations pretty easy.
+
+  * Automatic schema migration: adding a new column to a persistent record is as simple as adding
+    the new field to the POJO and incrementing the `SCHEMA_VERSION_NUMBER` constant.
+  * Assisted schema migration: dropping, renaming and retyping columns is very easy, and more
+    sophisticated custom migrations can also be easily incorporated into Depot's schema migration
+    system.
+  * Data migration: migrations that do not change record schemas but manipulate their data can also
+    be registered and Depot will ensure that they run successfully and only once.
+  * Distributed migration coordination: Depot is designed so that you can bring up a dozen
+    application servers on a dozen machines and during their initialization they will coordinate
+    (through the database) which server will handle each migration and the other servers will block
+    any database access until those migrations have successfully completed.
+
+The addition of columns is automatic. Dropping, renaming and retyping are very simple:
+
+```java
+public DecorRepository (PersistenceContext ctx)
+{
+    super(ctx);
+
+    ctx.registerMigration(DecorRecord.class, 
+                          new SchemaMigration.Rename(17004, "scale", DecorRecord.ACTOR_SCALE));
+    ctx.registerMigration(DecorRecord.class, new SchemaMigration.Drop(17004, "offsetX"));
+    ctx.registerMigration(DecorRecord.class, new SchemaMigration.Drop(17004, "offsetY"));
+    ctx.registerMigration(DecorRecord.class, 
+                          new SchemaMigration.Retype(17004, DecorRecord.FURNI_SCALE));
+}
+```
+
+More complex migrations are also possible, one has to take care if they wish to preserve database
+agnosticism:
+
+```java
+ctx.registerMigration(FooRecord.class, new SchemaMigration(42) {
+    @Override
+    public Integer invoke (Connection conn, DatabaseLiaiason liaison) throws SQLException {
+        // go crazy with your raw JDBC connection or use the DatabaseLiaison to
+        // help you do things in a database agnostic way
+    }
+});
+```
+
+## Data Migration
+
+TBD