Add mention of installing shards when compiling from source

[jwoertink]

If someone new to Crystal wants to build from source, they will go here https://crystal-lang.org/install/from_sources/ and follow these instructions. Once done, they may want to start working shards, but won't really know where to start. I think adding a simple note to the bottom that mentions something like

If you want to install dependencies for your project, you'll need the shards command. You can see install instructions [here]

This would only be on the installing from source page since the shards command generally comes packaged when installing from package managers.