add useful resources for unity/xamarin support

src/csharp/experimental/README.md

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+This directory contains useful resources for getting gRPC C# to work on
+not-yet-supported platforms.
+
+# Unity & Xamarin
+gRPC C# currently doesn't support Unity or Xamarin, but some proof-of-concept
+work has been done. Some of the resources are shared in this directory to
+ease community work on Unity & Xamarin support.
+
+## Crosscompiling `grpc_csharp_ext` for Android
+
+* Install [Android NDK](https://developer.android.com/ndk/index.html)
+* Run `./build_native_ext_for_android.sh` to crosscompile using cmake.
+
+## Crosscompiling `grpc_csharp_ext` for iOS
+
+TBD

src/csharp/experimental/build_native_ext_for_android.sh

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+#!/bin/sh
+# Copyright 2018 gRPC authors.
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+# Helper script to crosscompile grpc_csharp_ext native extension for Android.
+
+cd "$(dirname "$0")/../../../cmake"
+
+mkdir -p build
+cd build
+
+# set to the location where Android SDK is installed
+ANDROID_NDK_PATH="$HOME/android-ndk-r16b"
+
+cmake ../.. \
+  -DCMAKE_SYSTEM_NAME=Android \
+  -DCMAKE_SYSTEM_VERSION=15 \
+  -DCMAKE_ANDROID_ARCH_ABI=armeabi-v7a \
+  -DCMAKE_ANDROID_NDK=${ANDROID_NDK_PATH} \
+  -DCMAKE_ANDROID_STL_TYPE=c++_static \
+  -DRUN_HAVE_POSIX_REGEX=0 \
+  -DRUN_HAVE_STD_REGEX=0 \
+  -DRUN_HAVE_STEADY_CLOCK=0 \
+  -DCMAKE_BUILD_TYPE=Release
+
+make grpc_csharp_ext