fk: Add install instructions

[seisman]

Description of proposed changes

Related to #39.

PRs for new software/tutorials

• Software metadata is complete
• The "last updated date" in the metadata page is up-to-date
• All links work
• You're listed as the author(s) in the metadata page
• Anyone who have made significant contributions (e.g., review) to
  the tutorials are listed in the meteda page

PRs for bugs/improvements

• You're listed as the proofreader in the metadata page

[github-actions]

This comment was written by the Continuous Documentation bot!

• Preview: https://seismo-learn.org/sitepreview/seismo-learn/software/fk-install
• Commit hash: 9d1f6e9