add guide for creating plc libraries

[abroooo]

This guide outlines the specialties of IEC61131 in comparison to languages like c/cpp and shall act as help when creating a plc interface for external libraries.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 91.01%. Comparing base (fedc06d) to head (299832f). Report is 11 commits behind head on master.

:exclamation: Current head 299832f differs from pull request most recent head 25cdf51

Please upload reports for the commit 25cdf51 to get more accurate results.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1213      +/-   ##
==========================================
- Coverage   95.62%   91.01%   -4.61%     
==========================================
  Files         150      153       +3     
  Lines       42657    44929    +2272     
==========================================
+ Hits        40789    40893     +104     
- Misses       1868     4036    +2168     

:umbrella: View full report in Codecov by Sentry.
:loudspeaker: Have feedback on the report? Share it here.