docs: Add a README

The main purpose of adding this now is to document where users can get
the patched GObject Builder tool required to build upkg.

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+This package contains upkg: a tool for working with Unreal Engine package
+files.  This is alpha-quality software, and there are almost certainly packages
+which cause the tool to choke.  That being said, the list and info functions
+should work with most packages from the original Unreal and Unreal Tournament
+games.  Other Unreal Engine games may or may not work.  Extracting music files
+should work.
+
+== Installation ==
+=== Prerequisites ===
+Upkg is written in C99, and therefore requires a somewhat modern C compiler.
+The only hard run-time dependency beyond those required by standard C is the
+glib-2.0 library, as upkg is based on the GObject type system.  Upkg can also
+optionally make use of the libmodplug library, but at the moment this is only
+used to identify module formats when extracting music files, and upkg has its
+own code to do this identification if modplug is not available.
+
+Sources checked out from Git (which is the typical case, since there are
+currently no actual releases of upkg) do not contain any of the generated files
+which would normally be included in the distribution tarballs.  As such, a
+number of additional tools are required.  Any version numbers listed represent
+minimum tested versions.  Using newer versions of the tools should work, but
+older versions should not be expected to.
+
+ * GNU Autoconf version 2.68
+ * GNU Automake version 1.11.1
+ * GNU Libtool version 2.4
+ * GObject Builder (GOB) 2.0.9999 (**)
+
+(**) Unfortunately, the current releases of GObject Builder are missing support
+for dynamic types which are used extensively by upkg.  Moreover, GOB appears
+dead upstream, so you will need to use a custom-patched version, available
+from
+
+  http://git.draconx.ca/?p=gob2.git