Fix documentation now that rclrs is on crates.io

[Carter12s]

Was doing a pass through the documentation to familiarize myself with the crate. Found multiple "*Note: once rclrs is published on crates.io" blurbs in docs/building.md

I do believe rclrs is now on crates.io and this documentation should be updated. Not familiar enough with the crate at the moment to make this changes, so flagging this issue, but I intend to take a stab at it myself later.

[esteve]

@Carter12s thanks for filing this ticket. I believe #379 should address this issue, could you have a look at it when you have a moment? Thanks.