Translate howto.md to spanish

[davorpa]

What does this PR do?

Improve repo

For resources

Description

Translate to spanish: https://github.com/EbookFoundation/free-programming-books/blob/master/HOWTO.md

Why is this valuable (or not)?

It completes the language translation of transversal repo files to Spanish: #5449 #5117

How do we know it's really free?

N/A

For book lists, is it a book? For course lists, is it a course? etc.

N/A

Checklist:

• Read our contributing guidelines
• Search for duplicates. Translate CONTRIBUTING.md to spanish #5449 Code of conduct in Spanish #5117
• Include author(s) and platform where appropriate.
• Put lists in alphabetical order, correct spacing.
• Add needed indications (PDF, access notes, under construction)

Followup

• Check the output of Travis-CI for linter errors!