Add README to project root (#1253)

* Added root README.md draft

* Modified Contribution section to reflect unified guidelines

* Update README.md

Co-authored-by: Squirrel <gilescope@gmail.com>

* Expand Substrate description

* Added Badges and Upstream deps

* Fixed badge links

* Fixed CONTRIBUTING.md links, added security and resources section to root README.md

* Moved runtimes link to Polkadot section

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Co-authored-by: Squirrel <gilescope@gmail.com>
Co-authored-by: Oliver Tale-Yazdi <oliver.tale-yazdi@parity.io>