ct: needs better getting-started instructions about downloads and verification

[naisanza]

From the documentation it refers to using ./bin/ct but it does not exist

https://github.com/coreos/container-linux-config-transpiler/tree/master/doc

[lucab]

More specifically, getting-started.md is missing an introductory section where it should:

• link back to ct github repo
• specify that signed pre-built static binaries are available, linking to the releases page
• showing how to download, verify the sig and unpack them
• mentioning that ./bin/ct refers to the just-unpacked ct binary

For reference for @naisanza, signed binary artifacts are available at https://github.com/coreos/container-linux-config-transpiler/releases.