Update README to reflect the state of pycrypto

[alexmgr]

• Put forward that pycrypto is no longer maintained
• Mention usable alternatives up-front

[alexmgr]

As mentioned by others in previous issues, the state of pycrypto should be clear to people landing on the project page.
Given the current state of the project, people should be encouraged to use maintained alternatives.

In my view, the current undetermined state is harmful to both python users and crypto.

[enricobacis]

Thanks for the alternative @alexmgr, I didn't know about pycryptodome

[johnthagen]

Agreed, especially given there seems to exist at least one unpatched vulnerability: #176

[kekePower]

The issue I have is that PyCryptodome isn't a direct dropin replacement for PyCrypto and since I don't know much about Python, I have no means of patching packages.

/me is frustrated