Add Spanish quick-start README

[slegarraga]

Adds a short README.es.md covering the stable quick-start content, the install table, platform-specific permission notes, and the honest caveats from the English README. Adds the Spanish language link at the top of the main README.

Closes #16

[vstrelnikof]

Thank you — this is a genuinely well-made first contribution, and it lands the way issue #16 asked for: the stable parts only, no attempt to mirror the whole README into a second file that would rot.

A few things I checked, so you know it was read properly rather than waved through:

• The four installer filenames were verified against the actual assets on the v0.12.0 release, not just against the English README — they match exactly.
• The unsigned installers caveat survived the translation. That one matters more than it looks: we would rather a user meet the warning in the README than in a Gatekeeper dialog with no context, so a translation that quietly dropped it would have been a problem.
• Every link is relative and resolves; no hardcoded version numbers anywhere, which is what keeps this file from going stale on the next release.

Two follow-ups from me, neither of them anything you need to do:

1. I am restoring the docs/DECISIONS.md link in the Flatpak paragraph — it is where the reasoning lives and the conditions under which we would revisit it, and a reader who disagrees with the decision should be able to find the argument.
2. docs/RELEASING.md names the docs that block a release until they are in step. I am adding the translated READMEs to that list, so this file is maintained deliberately rather than by luck.

Issue #16 covers Ukrainian as well, so I am reopening it after the merge — this closes the Spanish half of it.

Unrelated to the PR, and only because it will follow you into every repository you commit to: your git config user.name has a stray curly quote in it, so the commit is authored by “Sebastian. Worth fixing locally when you get a moment.

Merging now. Thanks again.