Bump version to 0.3.0 and add alpha version disclaimer

[kevinsung]

No description provided.

[Strilanc]

Could you verify that when we do a dev release of openfermion cirq, that pip install openfermioncirq doesn't pick the dev release? If it does the wrong thing, we need to remove the a.

[kevinsung]

pip picks up the dev release. Let's remove the a, and in fact, just move to what we do at Cirq, only marking the dev versions as pre-releases.

[kevinsung]

I also copied the alpha disclaimer from Cirq just to make things clearer.