Update Readme of the `polkadot` crate (#5326)

- Typos.
- Those telemetry links like https://telemetry.polkadot.io/#list/Kusama
didn't seem to properly point to a proper list (anymore?) - updated
them.
- Also looks like it was trying to use rust-style linking instead of
markdown linking, changed that.
- Relative links do not work on crates.io - updated to absolute,
similarly as some already existing links, such as contribution
guidelines.