message: docs(README): add debian repository in one line
  For convenience, create the apt source file and add the repository in one line.
diff: @@ -41,10 +41,10 @@ installers for all supported operating systems.
  #### Debian and Ubuntu based Package Repository (GNU/Linux x86/x64)
  -1. Save the following as `/etc/apt/sources.list.d/etcher.list`:
  +1. Add Etcher debian repository:
  ```
  - deb https://dl.bintray.com/resin-io/debian stable etcher
  + echo "deb https://dl.bintray.com/resin-io/debian stable etcher" | sudo tee /etc/apt/sources.list.d/etcher.list
  ```
  2. Trust Bintray.com's GPG key:
  