changed default branch to `rolling`

[JanStaschulat]

• [x] - depricated master branch: added notice in README
• [x] - changed default branch of rclc repository to rolling

[codecov-commenter]

Codecov Report

Merging #376 (234ba74) into master (97ab70c) will not change coverage. The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #376   +/-   ##
=======================================
  Coverage   69.63%   69.63%           
=======================================
  Files          16       16           
  Lines        2760     2760           
  Branches      766      766           
=======================================
  Hits         1922     1922           
  Misses        452      452           
  Partials      386      386           

[JanStaschulat]

Hi @pablogs9, I updated the README. Could you approve, if its okay for you?