Update SignedAPKAndroid.md

Summary:
Update docs to prevent build process from failing when collaborators do not have MYAPP_RELEASE_STORE_FILE in their env.

Build process often fails when following the existing documentation on setting the signingConfigs in build.gradle, as not all collaborators have the required ENV set in their gradle.properties
Closes facebook#11716

Differential Revision: D4389150

Pulled By: hramos

fbshipit-source-id: 37941382950fac41f19d19317f49769588b135b4

Author: alvinthen

docs/SignedAPKAndroid.md

@@ -54,10 +54,12 @@ android {
     defaultConfig { ... }
     signingConfigs {
         release {
-            storeFile file(MYAPP_RELEASE_STORE_FILE)
-            storePassword MYAPP_RELEASE_STORE_PASSWORD
-            keyAlias MYAPP_RELEASE_KEY_ALIAS
-            keyPassword MYAPP_RELEASE_KEY_PASSWORD
+            if (project.hasProperty('MYAPP_RELEASE_STORE_FILE')) {
+                storeFile file(MYAPP_RELEASE_STORE_FILE)
+                storePassword MYAPP_RELEASE_STORE_PASSWORD
+                keyAlias MYAPP_RELEASE_KEY_ALIAS
+                keyPassword MYAPP_RELEASE_KEY_PASSWORD
+            }
         }
     }
     buildTypes {