Update README.md with missing modules, updated contributors and cosmetic changes

[jonesmartins]

Closes #91.

I'm not sure about what qualifies as a module contributor. I considered a contributor someone who changed some module's operators or its overrides. I didn't consider typo fixes or comment fixes as enough a contribution to add to the table, but it's a worthy contribution nonetheless :-)

The two last commits change a few module descriptions. I standardized Ext modules' descriptions to "Additional operators on ... (e.g. ...)". Operator examples might not be a good idea, which is why I added them last. They might help orient users when searching for these operations (or they might confuse them even more).

Two suggestion for issues and contributions:

• To avoid having to remember to update the README from time to time, one could add a PR template encouraging users to add their contributions to the README.md;
• Add a GitHub action to ensure all URLs are valid;

[lemmy]

Can you fix the DCO check?

[jonesmartins]

Fixed!