Adding generic installation instructions to the beginning of the install guilde

There still needs to be lots more content here. Please add it. In particular,
once we have the download tool, it would be good to have screenshots of that
with instructions.

Also, we probably want to show screenshots of downloading and expanding the
zip file not from the CLI.

Change-Id: I5ce1cf3ee314fc5ad5e6311d582a95bdf2a9d40c
Signed-off-by: Colin Dixon <colin@colindixon.com>
Merge "Adding generic installation instructions to the beginning of the install guilde"