Update the `CONTRIBUTING.md` in order to avoid any new merging problems in the future!

[patricekrakow]

[patricekrakow]

After some analysis, I think we did nothing wrong :-)

We can just

• merge feature/inventory-change-4-0-0 into feature/errors-3-0-0-Lars, and later
• merge feature/errors-3-0-0-Lars into develop.

[larsolofsson]

I want to discussed this issue. Why couldn't it be merged?
We couldn't

• merge feature/inventory-change-4-0-0 into feature/errors-3-0-0-Lars

I had to copy changes from feature/inventory-change-4-0-0 to feature/errors-3-0-0-Lars,

[patricekrakow]

I think this was a "false problem": we could have merged the two feature branches into the develop branch in whatever order! Nothing needs to be added to the CONTRIBUTING.md file. I will prepare a Git 101 session.