Update README.

README

@@ -1,7 +1,44 @@
-The source code repository for Druntime is:
-https://github.com/D-Programming-Language/druntime
+DRuntime: Runtime Library for the D Programming Language
+========================================================
 
-Druntime is the minimum library required to support the D programming
-language. It includes the system code required to support the garbage
-collector, associative arrays, exception handling, array vector operations,
-startup/shutdown, etc.
+This is DRuntime. It is the low-level runtime library
+backing the D programming language.
+
+DRuntime is typically linked together with Phobos in a
+release such that the compiler only has to link to a
+single library to provide the user with the runtime and
+the standard library.
+
+Purpose
+-------
+
+DRuntime is meant to be an abstraction layer above the
+compiler. Different compilers will likely have their
+own versions of DRuntime. While the implementations
+may differ, the interfaces should be the same.
+
+Features
+--------
+
+The runtime library provides the following:
+
+* The Object class, the root of the class hierarchy.
+* The associative array implementation.
+* Type information and RTTI.
+* Common threading and fiber infrastructure.
+* Synchronization and coordination primitives.
+* Exception handling and stack tracing.
+* Garbage collection interface and implementation.
+* Program startup and shutdown routines.
+* Low-level math intrinsics and support code.
+* Interfaces to standard C99 functions and types.
+* Atomic load/store and binary operations.
+* CPU detection/identification for x86.
+* System-independent time/duration functionality.
+* D ABI demangling helpers.
+* Low-level bit operations/intrinsics.
+
+Licensing
+---------
+
+See the LICENSE file for licensing information.