Add make serve-local command to Makefile

[alexsoble]

What does this do?

Adds an additional command to the Makefile for quickly serving the API locally:

make serve-local

Why?

• Makes the Makefile more comprehensive in terms of the commands it offers users.
• Makes the command to serve the app discoverable via the Makefile.
• Saves the user from remembering a custom-named command when getting started with a new OpenFisca project.

Notes

• The Makefile runs the serve command without any arguments as a form of quickstart; a user would need to run the full openfisca serve command if they want to send it command line arguments.
• Named the command serve-local under the assumption that users will want different config when serving in production.

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Minor change. These changes:

• Change non-functional parts of this repository (for instance editing the README).

[alexsoble]

☝️ A short note on motivation — as I've spun up a few prototype OpenFisca projects recently, I've found a make serve-local shortcut useful for getting started faster.

Opening this PR in case other newer folks would find this useful too!

[bonjourmauko]

Thanks !