Add README explaining how to build WebView.

Explain how to build WebView now that building it as part of AOSP is no
longer supported, and document the build settings used to produce the
current prebuilts.

Bug: 22593928
Change-Id: I4c59a19eff86f1b2ad742a3df56e82e067872864
diff --git a/README b/README
new file mode 100644
index 0000000..57d6286
--- /dev/null
+++ b/README
@@ -0,0 +1,25 @@
+Building the Chromium-based WebView in AOSP is no longer supported. WebView can
+now be built entirely from the Chromium source code.
+
+General instructions for building WebView from Chromium:
+https://www.chromium.org/developers/how-tos/build-instructions-android-webview
+
+------
+
+The prebuilt libwebviewchromium.so included in these APKs is built from Chromium
+release tag 44.0.2403.114. To match our build settings, set:
+
+GYP_DEFINES="OS=android release_unwind_tables=0 buildtype=Official chrome_build_id='2e7c7a0f-c733-4b9a-9c8e-d3cf0938369b'"
+
+in your environment before running build/gyp_chromium.
+
+------
+
+Due to WebView API changes in the M release, the Java code in the Chromium 2403
+branch is not compatible with M. We'll be working on upstreaming the M-specific
+Java changes to Chromium once the final M SDK is released.
+
+------
+
+For questions about building WebView, please see
+https://groups.google.com/a/chromium.org/forum/#!forum/android-webview-dev