Update documentation about number_of_handles

[JanStaschulat]

Often developers are not aware to configure xrce-dds middleware also when initializing the Executor with number_of_handles. I added docu in the README.md and executor.h

related issue: https://github.com/ros2/rclc/issues/325

[JanStaschulat]

@mergify backport humble foxy

[mergify[bot]]

backport humble foxy

✅ Backports have been created

* [#327 Update documentation about number_of_handles (backport #326)](https://github.com/ros2/rclc/pull/327) has been created for branch `humble` * [#328 Update documentation about number_of_handles (backport #326)](https://github.com/ros2/rclc/pull/328) has been created for branch `foxy`

[codecov-commenter]

Codecov Report

Merging #326 (f553967) into master (361fcdf) will decrease coverage by 0.15%. The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #326      +/-   ##
==========================================
- Coverage   69.20%   69.06%   -0.15%     
==========================================
  Files          16       16              
  Lines        2715     2715              
  Branches      765      765              
==========================================
- Hits         1879     1875       -4     
- Misses        450      452       +2     
- Partials      386      388       +2     

Impacted Files	Coverage Δ
rclc/src/rclc/action_goal_handle.c	58.59% <0.00%> (-2.55%)	:arrow_down:

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