Clarify Shaderc wraps around other tools

Describe why it exists: what does Shaderc add on top?

Say that Shaderc ships in the Android NDK. Point to the AOSP repos
as the authoritative sources of what is really in the NDK.  Also say
that those repos are downstream from the GitHub repos.

1 files changed, 17 insertions, 1 deletions

diff --git a/README.md b/README.md
index a1270f5..f322636 100644
--- a/README.md
+++ b/README.md
@@ -10,9 +10,25 @@ At the moment it includes:
 - [`glslc`](glslc), a command line compiler for GLSL to SPIR-V, and
 - `libshaderc` a library API for doing the same.
 
+Shaderc wraps around core functionality in
+[Glslang](https://github.com/KhronosGroup/glslang)
+and [SPIRV-Tools](https://github.com/KhronosGroup/SPIRV-Tools).  Shaderc aims to
+to provide:
+* a command line compiler with GCC- and Clang-like usage, for better
+  integration with build systems
+* an API where functionality can be added without breaking existing clients
+* an API supporting standard concurrency patterns across multiple
+  operating systems
+* increased functionality such as file `#include` support
+
 ## Status
 
-Shaderc is experimental, and subject to significant incompatible changes.
+Shaderc is approaching maturity, but is still subject to incompatible changes.
+
+Shaderc has been shipping in the
+[Android NDK](https://developer.android.com/ndk/index.html) since version r12b.
+(The NDK build uses sources from https://android.googlesource.com/platform/external/shaderc/.
+Those repos are downstream from GitHub.)
 
 For licensing terms, please see the [`LICENSE`](LICENSE) file.  If interested in
 contributing to this project, please see [`CONTRIBUTING.md`](CONTRIBUTING.md)