[Android] Separate playground (#2688)

* [Android] Separate Playground into https://github.com/apache/incubator-weex-playground

.gitmodules

@@ -0,0 +1,4 @@
+[submodule "weex-playground"]
+	path = weex-playground
+	url = https://github.com/apache/incubator-weex-playground.git
+	branch = master

android/README.md

@@ -3,4 +3,21 @@
   See https://gradle.org/install#with-a-package-manager
 ## Build Weex   
 run `gradle build`   
-Playground app artifact will be under 'playground/app/build'
+Playground app artifact will be under 'playground/build'
+## The submodule weex-playground
+[Weex Playground](https://github.com/apache/incubator-weex-playground) can be used to test and preview weex pages on Android and it has been added as a submodule.
+### Relationship between weex and playground
+* The playground submodule will be cloned and updated automatically when developers clone the ‘incubator-weex’ repo and build it.In this case, playground project will implement the source code of weex-sdk.
+* When developer directly clone the 'weex-playground' repo and run it separately, the playground project will implement a corresponding aar version of weex-sdk.
+### When to publish SNAPSHOT
+When some commits of  playground rely on unpublished features of weex-sdk,a SNAPSHOT of weex-sdk will be needed to support it.
+### How to publish SNAPSHOT
+ `cd android `  
+`./gradlew clean install ArtifactoryPublish -PweexVersion=$PUBLISH_VERSION -PbintrayUser=alibabaweex -PbintrayApiKey=$JCENTER_TOKEN `  
+
+* Explanation for variable:  
+$PUBLISH_VERSION The version of snapshot, like 0.26.1.3-SNAPSHOT  
+$JCENTER_TOKEN The private key for JCenter (https://bintray.com/alibabaweex/maven/weex_sdk/), which is the distribution channel for Android
+
+
+

android/build.gradle

@@ -49,18 +49,20 @@ subprojects {
             }
         }
     }
-    ext {
-        compileSdkVersion=26
-        buildToolsVersion="26.0.3"
-        minSdkVersion=14
-        targetSdkVersion=26
-        supportLibVersion="26.0.2"
-        fastjsonLibVersion="1.1.46.android"
-        //Default value for buildCpp is true
-        buildCpp = !project.hasProperty("buildCpp") || buildCpp.equals("true")
-        //Default value for disableCov is false
-        disableCov = project.hasProperty("disableCov") && disableCov.equals("true")
-    }
+}
+ext {
+    compileSdkVersion=26
+    buildToolsVersion="26.0.3"
+    minSdkVersion=14
+    targetSdkVersion=26
+    supportLibVersion="26.0.2"
+    fastjsonLibVersion="1.1.46.android"
+    //Default value for buildCpp is true
+    buildCpp = !project.hasProperty("buildCpp") || buildCpp.equals("true")
+    //Default value for disableCov is false
+    disableCov = project.hasProperty("disableCov") && disableCov.equals("true")
+    implementFromWeex = true
+
 }
 
 //task clean(type: Delete) {