Update README.md - java 8 required (fixes #127) (#128)

ref: syncthing/syncthing-android@61e4536

README.md

@@ -42,6 +42,7 @@ The project is translated on [Transifex](https://www.transifex.com/projects/p/sy
 `Make sure, Python is installed and available on the PATH environment variable.`
 - Git (for Linux) or Git for Windows
 `Make sure, git (or git.exe) is installed and available on the PATH environment variable. If Git is not found on the PATH environment variable, build-syncthing.py will automatically try to download and setup MinGit 2.19.0-x64 on the PATH.`
+- Java Version 8 (you might need to set `$JAVA_HOME` accordingly)
 
 ### Build instructions