add build documents from source code

[melin]

add build documents from source code to help users quickly use

[rdblue]

Do you mean how to invoke the build? If so, you can run this to build and run tests:

./gradlew build

If you want to skip tests, run:

./gradlew build -x test

[rdblue]

Would you like to open a PR to add those to the README?

[timmylicheng]

Just started to get into iceberg. I figured to do a PR for this: #304

[timmylicheng]

#304 is merged in.