Add links to the hosted versions of the docs.

Change-Id: I806f1633909ad816622a3019c6153434fdf8bcd2

README.md

 Android Native Development Kit (NDK)
 ====================================
 
+The latest version of this document is available at
+https://android.googlesource.com/platform/ndk/+/master/README.md.
+
 **Note:** This document is for developers _of_ the NDK, not developers
 that use the NDK.
 

docs/GeneratingSysroots.md

 Generating NDK Sysroots
 =======================
 
+The latest version of this document is available at
+https://android.googlesource.com/platform/ndk/+/master/docs/GeneratingSysroots.md.
+
 The `platforms/` subtree of an NDK package contains sysroots for each
 architecture and API level. The contents of these sysroots are generated as part
 of the NDK build.

docs/Testing.md

 Testing the NDK
 ===============
 
+The latest version of this document is available at
+https://android.googlesource.com/platform/ndk/+/master/docs/Testing.md.
+
 Testing Tools
 -------------
 

docs/Toolchains.md

 Working with the NDK Toolchains
 ===============================
 
+The latest version of this document is available at
+https://android.googlesource.com/platform/ndk/+/master/docs/Testing.md.
+
 The toolchains shipped in the NDK are not built as a part of the NDK build
 process. Instead they are built separately and checked into git as prebuilts
 that are repackaged when shipped in the NDK. This applies to both Clang and GCC.