[doc request] modify the README.md to cover more details for ops

[allencloud]

Hi, there,

I found that d2p has no sufficient guidance for enterprise operators. This is really an obstacle for ops to take advantage of d2p.

So, I am wondering if we could add more guidance of doc in README.md including:

• What is d2p-migrator;
• Why is d2p;
• How to use d2p（commandline instructions，detailed usage guidance）;
• the Architecture of d2p
• any other essential parts

[HusterWan]

@allencloud thanks for report, I will modify the README.md this week .