Adding 'CLion integration' to clang-format doc

Summary:
This commit adds a chapter 'CLion integration' to ClangFormat.rst. 
The official announcement of clang-format support in CLion 2019.1: https://blog.jetbrains.com/clion/2019/03/clion-2019-1-embedded-dev-clangformat-memory-view/

Patch by Marina Kalashina 

Reviewers: djasper, sylvestre.ledru, krasimir

Reviewed By: sylvestre.ledru

Subscribers: jdoerfert, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D60115

llvm-svn: 357566