*: Update top-level readme

Added links to full stacks published by Google.
Updated formatting slightly.
Added note about docs/ dir

Change-Id: Iaf62827a2d31f8b57321dec41ef81d4f56af79e0
Reviewed-on: https://bluetooth-review.git.corp.google.com/c/bluetooth/+/2100
Reviewed-by: Ani Ramakrishnan <aniramakri@google.com>

README.md

diff --git a/README.md b/README.md
index 896b000..f8972dd 100644
--- a/README.md
+++ b/README.md
@@ -2,14 +2,23 @@
 
 ## What is this?
 
-This is a repository for shared source libraries that can be useful for
-multiple Bluetooth stacks.  Many of these are meant to be stack-agnostic and
-platform-agnostic and implement a useful part of a published Bluetooth
-specification.
+This is a repository for shared libraries and tools that can be useful for
+multiple Bluetooth stacks.
+
+These tools and libraries are meant to be:
+ * Stack-agnostic and platform-agnostic
+ * Implement a useful subset of a published Bluetooth specification.
 
 It has been started as a repository for developing Rust crates but can be
-easily expanded to include libraries for other languages.
+easily expanded to include libraries and tools written in other languages.
+
+## What is this not?
+
+It is not a full Bluetooth stack.  Google has two officially open-source Bluetooth stacks:
+ * [Fluoride](https://cs.android.com/android/platform/superproject/main/+/main:packages/modules/Bluetooth/) - Widely used in AOSP
+ * [Sapphire](https://cs.opensource.google/fuchsia/fuchsia/+/main:src/connectivity/bluetooth/) - Used in Fuchsia
 
 ## Structure
 
  - rust - Rust Crates
+ - docs - General documentation (documentation for each library/tool is included in their directories)