README (#28533)

Summary:
Copy of android.md from the site + information about Nightly builds

It's a bit of duplication with separate repo pytorch.github.io , but I think more people will find it and we can faster iterate on it and keep in sync with the code.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/28533

Reviewed By: dreiss

Differential Revision: D18153638

Pulled By: IvanKobzarev

fbshipit-source-id: 288ef3f153d8e239795a85e3b8992e99f072f3b7

android/README.md

@@ -0,0 +1,118 @@
+# Android
+
+## Demo applications and tutorials
+
+Demo applications with code walk-through can be find in [this github repo](https://github.com/pytorch/android-demo-app).
+
+## Publishing
+
+##### Release
+Release artifacts are published to jcenter:
+
+```
+repositories {
+    jcenter()
+}
+
+dependencies {
+    implementation 'org.pytorch:pytorch_android:1.3.0'
+    implementation 'org.pytorch:pytorch_android_torchvision:1.3.0'
+}
+```
+
+##### Nightly
+
+Nightly(snapshots) builds are published every night from `master` branch to [nexus sonatype snapshots repository](https://oss.sonatype.org/#nexus-search;quick~pytorch_android)
+
+To use them repository must be specified explicitly:
+```
+repositories {
+    maven {
+        url "https://oss.sonatype.org/content/repositories/snapshots"
+    }
+}
+
+dependencies {
+    ...
+    implementation 'org.pytorch:pytorch_android:1.4.0-SNAPSHOT'
+    implementation 'org.pytorch:pytorch_android_torchvision:1.4.0-SNAPSHOT'
+    ...
+}
+```
+The current nightly(snapshots) version is the value of `VERSION_NAME` in `gradle.properties` in current folder, at this moment it is `1.4.0-SNAPSHOT`.
+
+## Building PyTorch Android from Source
+
+In some cases you might want to use a local build of pytorch android, for example you may build custom libtorch binary with another set of operators or to make local changes.
+
+For this you can use `./scripts/build_pytorch_android.sh` script.
+```
+git clone https://github.com/pytorch/pytorch.git
+cd pytorch
+sh ./scripts/build_pytorch_android.sh
+```
+
+The workflow contains several steps:
+
+1\. Build libtorch for android for all 4 android abis (armeabi-v7a, arm64-v8a, x86, x86_64)
+
+2\. Create symbolic links to the results of those builds:
+`android/pytorch_android/src/main/jniLibs/${abi}` to the directory with output libraries
+`android/pytorch_android/src/main/cpp/libtorch_include/${abi}` to the directory with headers. These directories are used to build `libpytorch.so` library that will be loaded on android device.
+ 
+3\. And finally run `gradle` in `android/pytorch_android` directory with task `assembleRelease`
+
+Script requires that Android SDK, Android NDK and gradle are installed.
+They are specified as environment variables:
+
+`ANDROID_HOME` - path to [Android SDK](https://developer.android.com/studio/command-line/sdkmanager.html)
+
+`ANDROID_NDK` - path to [Android NDK](https://developer.android.com/studio/projects/install-ndk)
+
+`GRADLE_HOME` - path to [gradle](https://gradle.org/releases/)
+
+
+After successful build you should see the result as aar file:
+
+```
+$ find pytorch_android/build/ -type f -name *aar
+pytorch_android/build/outputs/aar/pytorch_android.aar
+pytorch_android_torchvision/build/outputs/aar/pytorch_android.aar
+libs/fbjni_local/build/outputs/aar/pytorch_android_fbjni.aar
+```
+
+It can be used directly in android projects, as a gradle dependency:
+```
+allprojects {
+    repositories {
+        flatDir {
+            dirs 'libs'
+        }
+    }
+}
+
+android {
+    ...
+    packagingOptions {
+        pickFirst "**/libfbjni.so"
+    }
+    ...
+}
+
+dependencies {
+    implementation(name:'pytorch_android', ext:'aar')
+    implementation(name:'pytorch_android_torchvision', ext:'aar')
+    implementation(name:'pytorch_android_fbjni', ext:'aar')
+}
+```
+
+At the moment for the case of using aar files directly we need additional configuration due to packaging specific (`libfbjni.so` is packaged in both `pytorch_android_fbjni.aar` and `pytorch_android.aar`).
+```
+packagingOptions {
+    pickFirst "**/libfbjni.so"
+}
+```
+
+## More Details
+
+You can find more details about the PyTorch Android API in the [Javadoc](https://pytorch.org/docs/stable/packages.html).