Add resource: Covalent

[martymonero]

Resources

• Link: https://chempirical.com/covalent/
  Why it helps students: it lets students draw structural formulas in any browser, Chromebooks and tablets included, checks valences while they draw and labels R/S and E/Z centres, so a wrong structure shows up before it goes into homework or a lab report.

I build Covalent, so please weigh this accordingly. It is free, needs no account or installation and shows no ads. It exports SMILES, MOL, SVG and PNG.

Checklist

• Each entry added in the correct section, in alphabetical order (case-insensitive)
• Each entry follows the format from CONTRIBUTING.md: - **[Name](https://homepage)** - Short description.
• Each entry meets the Quality Standards: real, maintained, reputable, pricing noted
• No entry duplicates an existing one in the same section

[github-actions]

Thanks for your first PR! A couple of things that'll speed up review: place your entry in its correct alphabetical position, and make sure it meets the Quality Standards (real, maintained, reputable, with pricing noted). CI will check the entry format and alphabetical order automatically, see CONTRIBUTING.md for the full checklist.