Changing "echo" to "printf" in the readme when explaining how to generate a new key

The default key is

    key = 'eb0a191797624dd3a48fa681d3061212'

Under the current linux mint (ubuntu), the proper key is generated using printf but not echo (newline in the string):

    $ echo "master" | md5sum
    c963080767f45828c31f83ca5cd25d36  -
    $ printf "master" | md5sum
    eb0a191797624dd3a48fa681d3061212  -
Merge pull request #1 from twitwi/change-echo-to-printf-in-readme

Changing "echo" to "printf" in the readme when explaining how to generate a new key