update polkadot, substrate, cumulus readme (#1182)
* update readmes * temporary ReadMe for the Polkadot SDK * delete welcome readme * update links on substrate readme * update links on polkadot readme * update links on cumulus readme * update overseer feature comment Co-authored-by: Liam Aharon <[email protected]> * Update cumulus/README.md Co-authored-by: Liam Aharon <[email protected]> * Update cumulus/README.md Co-authored-by: Liam Aharon <[email protected]> * Update polkadot/README.md Co-authored-by: Liam Aharon <[email protected]> * Update polkadot/README.md Co-authored-by: Liam Aharon <[email protected]> * Update polkadot/README.md Co-authored-by: Liam Aharon <[email protected]> * update gitlab links Co-authored-by: Liam Aharon <[email protected]> * terminal friendly convention --------- Co-authored-by: Liam Aharon <[email protected]>