- Mention that the instructions are written for Bash.
- Quote the CMAKE_INSTALL_PREFIX to handle spaces in $(pwd).
- Remove SSH clone instructions. Instructions on how to clone are for
  those unfamiliar with GitHub, so fewer options are better and SSH
  requires additional configuration to make work.
- Replace ninja with make, because more people are familiar with it and
  we probably don't need to explain how to install it. Fixes #956.
- Remove the section mentioning cygwin, as it's unclear and doesn't
  seem necessary.
- Mention minimum language version in the Dependencies section.
- Move `cd $BUILD_DIR` out of the Linux configuration section because
  it's needed on both platforms. Add a line about creating the build
  directory, too.