docs: explain building a plugin together with others

[locker95]

Summary

When you're in a plugin directory and want Caddy with your plugin and other plugins, xcaddy --with other/plugin run fails with unknown flag: --with.

That's expected: without the build subcommand, args are passed through to Caddy. --with is a xcaddy build flag.

This documents the working pattern:

xcaddy build \
  --with github.com/me/my-plugin=. \
  --with github.com/mholt/caddy-events-exec
./caddy run

Docs only — same approach maintainers already described in #227.

Test plan

• README spells out that --with is for xcaddy build, not xcaddy run
• Example uses =. for the local plugin plus a second --with
• Skim the new subsection under "For plugin development"

Fixes #227