Add Codespaces to the repository (#180)

As discussed with @Kianenigma, @PierreBesson

 and others, this pull
request brings the ability to spin up a Codespace instance inside this
repository. The PR has two components:

- `devContainer.json`: The setup instructions to initialize the instance
with
- `updateContentCommand.sh`: A custom shell script to install the necessary
dependencies for Substrate and the contracts node template on the
Codespace instance that is invoked immediately after the instance is
created

It also includes a badge in the README to allow users to start a Codespace instance using these settings from their own GitHub account.

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Co-authored-by: Ben Greenberg <[email protected]>
Co-authored-by: German <[email protected]>
Co-authored-by: Sacha Lansky <[email protected]>
Co-authored-by: Sasha Gryaznov <[email protected]>