Improve docs around srcgen

Change-Id: Ib64e69a374abd551c1212ea801799e4834d2a063
diff --git a/README.android b/README.android
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+This repo contains Android's copy of ICU4C and ICU4J and associated tools and code.
+
+Directories
+===========
+
+icu4j/,icu4c/   - ICU4J and ICU4C with Google and Android-specific patches applied.
+
+android_icu4j/  - A transformed copy of icu4j source repackaged into android.icu.
+                  This is the copy of ICU4J used in the Android platform.
+                  IMPORTANT NOTE: Do not make changes to this code directly.
+                  See tools/README.android.
+
+android_utils/  - ICU4C runtime initialization / Android integration code.
+
+tools/          - Code / data maintenance tools. See tools/README.android.