Update README to point to authoritative docs.

The README was referencing an outdated source of information and
included many details that had become stale over time, as this wasn't
being updated when new prebuilts were provided.

Refer instead to the current authoritative documentation describing how
to build WebView for AOSP, and remove the specific details about how to
rebuild the prebuilt as they're prone to becoming outdated and are not
really necessary in any case - rebuilding the same version is not
typically desirable and the Chromium docs suggest appropriate build
arguments for the current version.

Fixes: 139300536
Test: n/a, doc change only
Change-Id: I486c6c436d77d021b36b3618fe763f1bbe7079c0
(cherry picked from commit 6174fba679427a2c141036089253efd48c6b0371)
diff --git a/README b/README
deleted file mode 100644
index e4d341b..0000000
--- a/README
+++ /dev/null
@@ -1,45 +0,0 @@
-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
-
-For questions about building WebView, please see
-https://groups.google.com/a/chromium.org/forum/#!forum/android-webview-dev
-
-------
-
-The prebuilt APKs here are built from Chromium upstream sources; check the
-commit messages to see the version number for a particular prebuilt (the version
-number looks like 74.0.3729.127 and this is also the name of the tag in the
-Chromium git repository).
-
-If you want to build your own WebView, you should build the latest stable
-version, not the version published here: newer versions have important security
-and stability improvements.
-
-However, if you want to reproduce the native library (libwebviewchromium.so)
-contained in these prebuilt APKs, you should be able to do so with the following
-GN arguments:
-
-target_os = "android"
-is_debug = false
-is_component_build = false
-is_official_build = true
-is_chrome_branded = false
-use_official_google_api_keys = false
-exclude_unwind_tables = true
-ffmpeg_branding = "Chrome"
-proprietary_codecs = true
-enable_remoting = true
-
-as well as specifying the appropriate target_cpu, which should be one of
-"arm", "arm64", "x86" or "x64".
-
-To build a complete WebView APK it is necessary to also set android_sdk_release
-to the current Android SDK version (for example, "q"). However, it's not always
-possible to do this successfully for all public Chromium versions: we are not
-able to publish the required Java code changes to support a given Android
-version until after that Android version's SDK has been publicly released, and
-so the public versions of Chromium do not always support the latest version of
-Android.
diff --git a/README.md b/README.md
new file mode 100644
index 0000000..a06bc0d
--- /dev/null
+++ b/README.md
@@ -0,0 +1,19 @@
+Building the Chromium-based WebView in AOSP is no longer supported or required.
+WebView can now be built entirely from the Chromium source code.
+
+Docs on how to build WebView from Chromium for use in AOSP are available here:
+https://chromium.googlesource.com/chromium/src/+/HEAD/android_webview/docs/aosp-system-integration.md
+
+For questions about building WebView, please contact our mailing list:
+https://groups.google.com/a/chromium.org/forum/#!forum/android-webview-dev
+
+---
+
+The prebuilt APKs provided here are built from Chromium upstream sources; you
+should check the commit message to see the version number for a particular
+prebuilt. The version number is formatted like "12.0.3456.789" and matches the
+tag in the Chromium repository it was built from.
+
+If you want to build your own WebView, you should generally build the latest
+stable version, not the version published here: newer versions have important
+security and stability improvements.