Open source the Sandbox plugin

Summary: This diff is mainly moving diffs around to open source the Sandbox plugin. I created the MD file writing some docs. The rest is just making our own FB implementation work

Reviewed By: danielbuechele

Differential Revision: D8731839

fbshipit-source-id: 27ac67223c6de9d1be406ab8c06b71b82d5407bc

docs/sandbox-plugin.md

@@ -0,0 +1,25 @@
+---
+id: sandbox-plugin
+title: Sandbox
+---
+
+The Sandbox plugin is useful for developers that had to test changes of their apps by pointing them to some Sandbox environment. Through this plugin and a few lines of code in the client,
+the app can get a callback and get the value that the user has input through Sonar. At this point, the developer can plugin its logic to save this setting in its app.
+
+## Setup
+
+To use the sandbox plugin, you need to add the plugin to your Sonar client instance.
+
+### Android
+
+```java
+import com.facebook.sonar.plugins.SandboxSonarPlugin;
+import com.facebook.sonar.plugins.SandboxSonarPluginStrategy;
+
+final SandboxSonarPluginStrategy strategy = getStrategy(); // Your strategy goes here
+client.addPlugin(new SandboxSonarPlugin(strategy));
+```
+
+### iOS
+
+Coming soon