Add contribution guide

[clhunsen]

With the integration of CONTRIBUTING files in the GitHub contribution process, we should set up such a guide.

What we should cover

• Coding style
  • use https://google.github.io/styleguide/Rguide.xml as a start
  • basically anything not matching the Google style reference (e.g., 4 spaces, line length longer than 80 characters allowed, identifiers only with . and not camel-case names, assignment only with =)
  • requireNamespace and package prefixes (e.g., igraph::)
  • function and method documentation in roxygen2 (see https://support.rstudio.com/hc/en-us/articles/200532317-Writing-Package-Documentation)
  • logging using the logging package
  • double quotes instead of single quotes
  • we use "networks", no "graphs"
  • we talk about authors, no developers
  • write TRUE instead of T (analogously for FALSE and F)
  • write tests!
  • ...
• Commit style
  • e.g., https://github.com/siemens/codeface/wiki/How-to-structure-commits
• Merge policy
  • tests must run successfully

Reference material

• https://github.com/opengovernment/opengovernment/blob/master/CONTRIBUTING.md
• https://github.com/siemens/codeface/wiki/Style-Guide
• https://google.github.io/styleguide/Rguide.xml

[clhunsen]

• [x] We should add the new date-handling functions to the coding guidelines (see #77).

[clhunsen]

With version v3.1, we now have a contribution guide and a changelog. :grinning: