Add link/badge to the forum

[mdtanker]

Description:

In the recent dev call we discussed making it easier to find the Fatiando forum page. We can add a link to the forum on the readme's and docs index pages for all the repositories.

We could either add text:

Ask a question!

or a badge:

Apply to:

• boule
• harmonica
• pooch
• verde
• ensaio
• dependente
• website
• tutorials
• community

Further instructions:

• Start by opening Pull Requests on each repository listed above.
• Optionally, we can open Issues on each repository if further discussion specific to that repository is needed.
• Mention this Issue on every Issue or Pull Request opened on each opened: Related to fatiando/community#XX
• Check-off the repository on the list above once the Pull Request is merged.
• Close this issue when all items are checked-off.

We want your help!

We know that maintenance tasks are very demanding, so we don't expect a single person to tackle this issue by themselves. Any help is very welcomed, so please comment below that you want to take care of the changes on any repository and we will assign it to you.

[leouieda]

👍🏾 great suggestions! How about the text version placed after the "Contact" link?