Add code style documentation

[rpanchyk]

No description provided.

[And1sS]

What do you think about adding this section?

### Data retrieval calls of same result
Try not to retrieve same data more than once:

// bad
if (getData() != null) {
   final Data resolvedData = resolveData(getData());

//  good
final Data data = getData();
if (data != null) {
   final Data resolvedData = resolveData(data);