docs: add a contributing guide and link it from the README and site

Nothing in the repo said that pull requests go to develop rather than master,
or that release-plz reads the commit subject to pick the next version. Both
cost a first-time contributor a round trip.

CONTRIBUTING.md covers those, the three checks CI runs, and the fact that the
e2e suites skip without a soulfind server. The README and the community page
link to it, and it joins the footer's Project column.

website/src/components/layout/footer.tsx

@@ -20,6 +20,7 @@ const COLUMNS: readonly FooterColumn[] = [
     links: [
       { label: "GitHub", href: LINKS.gh },
       { label: "Issues", href: LINKS.issues },
+      { label: "Contributing", href: LINKS.contributing },
       { label: "Releases", href: LINKS.releases },
       { label: "Changelog", href: LINKS.changelog },
     ],

website/src/lib/links.ts

@@ -3,6 +3,7 @@ export const LINKS = {
   issues: 'https://github.com/michel/soulseek-rs/issues',
   releases: 'https://github.com/michel/soulseek-rs/releases',
   changelog: 'https://github.com/michel/soulseek-rs/blob/master/CHANGELOG.md',
+  contributing: 'https://github.com/michel/soulseek-rs/blob/master/CONTRIBUTING.md',
   license: 'https://github.com/michel/soulseek-rs/blob/master/LICENSE',
   cratesClient: 'https://crates.io/crates/soulseek-rs',
   cratesLib: 'https://crates.io/crates/soulseek-rs-lib',

website/src/pages/community.tsx

@@ -239,10 +239,15 @@ export const Community = () => {
               open-source Soulseek server; run it locally to develop against a real protocol
               implementation without touching the public network.
             </p>
+            <p>
+              Pull requests go to <Code>develop</Code>; <Code>master</Code> only moves on a
+              release.
+            </p>
             <div className="mt-1 flex flex-wrap gap-2 sm:gap-3">
               <Button href={LINKS.issues} variant="accent">
                 Open an issue
               </Button>
+              <Button href={LINKS.contributing}>Contributing</Button>
               <Button href={LINKS.gh}>GitHub</Button>
             </div>
           </Prose>