Update README.md
AdMob: Update ti.admob to Google Play Services v20
Merge pull request #6 from muhammaddadu/TIMODOPEN-445

[TIMODOPEN-445] AdMob: Update ti.admob to Google Play Services v20
[TIMOB-17928] Updated iOS module for 64-bit

https://jira.appcelerator.org/browse/TIMOB-17928
Merge pull request #7 from pec1985/TIMOB-17928

[TIMOB-17928] Updated iOS module for 64-bit
[TIMODOPEN-454] AdMob: Update ti.admob to Google Play Services v21
Merge pull request #9 from muhammaddadu/TIMODOPEN-454

[TIMODOPEN-454] AdMob: Update ti.admob to Google Play Services v21
[TIMODOPEN-461] AdMob: Add isGooglePlayServicesAvailable method to module
Merge pull request #10 from muhammaddadu/TIMODOPEN-461

[TIMODOPEN-461] AdMob: Add isGooglePlayServicesAvailable method to module