Recommend to install with `--save-dev` when types are not part of the external API

[psqli]

The current generated README shows the install example with --save option:

DefinitelyTyped-tools/packages/publisher/src/generate-packages.ts

Line 176 in c1e60ab

lines.push(`> \`npm install --save ${typing.name}\``);

Wouldn't it be valuable to instruct users whether to choose --save or --save-dev for types?

For example:

If the types are consumed only within your package, install it with --save-dev option:

npm install --save-dev ...

If your package directly or indirectly exports the types, use the --save option instead of --save-dev.

[jakebailey]

We could change this, but I suspect changing the README will modify the content hash for each package and then cause us to republish all packages. (It'd be good to be wrong about that.)