Updated readme: add link to trial version

Author: phpbg

readme.md

@@ -19,6 +19,8 @@ Synchronize your phone with your WebDAV server, the easy way.
     <img width='90' alt='Get it on Google Play' src='./qr-playstore.png'/>
 </a>
 
+A free trial version (30 days) is also [available here](https://play.google.com/store/apps/details?id=com.phpbg.easysync.trial&utm_source=github&pcampaignid=pcampaignidMKT-Other-global-all-co-prtnr-py-PartBadge-Mar2515-1)
+
 ### On F-Droid
 <a href='https://f-droid.org/packages/com.phpbg.easysync'>
     <img width='240' alt="Get it on F-Droid" src='https://fdroid.gitlab.io/artwork/badge/get-it-on.png'>