docs: added spec requirement to plugin install

Author: ethanneff

README.md

@@ -73,7 +73,7 @@
     <!-- sample config.xml -->
     <widget id="com.eneff.branch.cordovatestbed" version="0.0.1" xmlns="http://www.w3.org/ns/widgets" xmlns:cdv="http://cordova.apache.org/ns/1.0">
       <!-- Branch -->
-      <plugin name="branch-cordova-sdk" />
+      <plugin name="branch-cordova-sdk" spec="^2.5.0" />
       <branch-config>
         <branch-key value="key_live_ndqptlgXNE4LHqIahH1WIpbiyFlb62J3" />
         <uri-scheme value="branchcordova" />
@@ -87,7 +87,7 @@
     <!-- sample config.xml -->
     <widget id="com.eneff.branch.cordovatestbed" version="1.0.0" xmlns="http://www.w3.org/ns/widgets" xmlns:gap="http://phonegap.com/ns/1.0">
       <!-- Branch -->
-      <plugin name="branch-cordova-sdk" />
+      <plugin name="branch-cordova-sdk" spec="^2.5.0" />
       <branch-config>
         <branch-key value="key_live_ndqptlgXNE4LHqIahH1WIpbiyFlb62J3" />
         <uri-scheme value="branchcordova" />