docs(readme): add installation and layout sections to README

README.md

@@ -2,3 +2,42 @@
 
 A collection of skills for cross-platform agentic development.
 
+## Install
+
+**Claude Code**
+
+```sh
+claude plugin marketplace add prdlk/skills
+claude plugin install skills@prdlk
+```
+
+**omp**
+
+```sh
+omp plugin marketplace add prdlk/skills
+omp plugin install skills@prdlk
+```
+
+**Codex**
+
+```sh
+codex plugin marketplace add prdlk/skills
+codex plugin add skills@prdlk
+```
+
+**opencode** (or any other agent, via [`skills`](https://github.com/vercel-labs/skills))
+
+```sh
+npx skills add prdlk/skills -g -a opencode
+```
+
+## Layout
+
+```
+.github/skills/<name>/SKILL.md     skills (source of truth)
+.agents/skills -> ../.github/skills  project discovery for Codex, opencode, omp
+.claude-plugin/marketplace.json    marketplace catalog for Claude Code, omp, Codex
+.claude-plugin/plugin.json         plugin manifest, points at .github/skills/
+```
+
+To add a skill, create `.github/skills/<name>/SKILL.md`. Its frontmatter `name` must match `<name>`.