[docs] Add contribution guidance section in document

[leonardBang]

We need add contribution guidance section in document, e.g. develop environment setting, code format, PR description
We can add it in readme.md and about.md

[MonsterChenzhuo]

Hi @leonardBang I am very interested in this，and I want do some job for flink-cdc，can I help to do that？
Thank you

[leonardBang]

Thanks @MonsterChenzhuo for taking this, I've assigned to you.

[leonardBang]

@MonsterChenzhuo you can refer #340 when you write the documentation

[MonsterChenzhuo]

@leonardBang Sorry, I forgot about this task some time ago due to job changes. Now, I have started this task and will submit pr soon

[gtk96]

@ruanhang1993 @GOODBOY008 @Jiabao-Sun I'm interested in it.