docs: added singleInstance for android

Author: ethanneff

README.md

@@ -92,6 +92,7 @@
         <host name="2d0s.app.link" scheme="https" />
         <host name="2d0s-alternate.app.link" scheme="https" />
       </branch-config>
+      <preference name="AndroidLaunchMode" value="singleInstance" />
     ```
 
   - PhoneGap
@@ -103,6 +104,7 @@
         <host name="2d0s.app.link" scheme="https" />
         <host name="2d0s-alternate.app.link" scheme="https" />
       </branch-config>
+      <preference name="AndroidLaunchMode" value="singleInstance" />
     ```
 
   - Change `com.eneff.branch.cordova`, `PW4Q8885U7`, `2d0s.app.link`, and `2d0s-alternate.app.link` to the values in your [Branch Dashboard](https://dashboard.branch.io/settings/link)