Update the branch for new features and enhancements in the "Contributions" section in the README.md

The second sentence in the "Contributions" sections in the Elytron README.md file currently says:

All new features and enhancements should be submitted to master only.

Since new features and enhancements should now be submitted against the 1.x branch instead of master, this sentence should be changed to:

All new features and enhancements should be submitted to the 1.x branch only.