Create RELEASE_NOTES.md

Hi,

I just want to have a release note to see what's new/what's will be new/what's is planned for the next version… So I create a Release Note (like the « what's new » for Mozilla Firefox project).  
For the next time you push to Play Store, please indicate too the Atarashii Github Repo (https://github.com/AnimaSA/Atarashii)  
And too, if you push an apk on github, you create a download/ section (See https://github.com/blog/665-apk-downloads-for-android-projects for more informations). It can be good for those who don't have the Play Store…

Thanks :-)