More specific instructions in the Contributing Guide for each task type

[leouieda]

In fatiando/.github#11, @MGomezN pointed out that we list contribution types at the start of the guide but then don't really provide specific instructions for each of these categories. The guide should be updated so that the instructions below match (and are linked from) the different contribution categories.

From that PR:

But, when breaking down the information, it is not very clear specific instructions for 📝 "Writing tutorials or examples". I was reading the instructions for "💡 Writing code for everyone to use" and that's why I was trying to install Make, rather than for the reminder itself.

This would make it easier for us to create a reminder list for PRs in code related repos that is better linked to the contributing guide.