Update installation instructions for Claude Code (#90)

Refactor installation instructions for installing the plugin so that it can be easy copied & pasted to the CLI, as Claude code expects each command separately.

README.md

@@ -23,18 +23,27 @@ Read the introduction: [Superpowers for Claude Code](https://blog.fsck.com/2025/
 
 ### Claude Code (via Plugin Marketplace)
 
+In Claude Code, register the marketplace first:
+
 ```bash
-# In Claude Code
 /plugin marketplace add obra/superpowers-marketplace
+```
+
+Then install the plugin from this marketplace:
+
+```bash
 /plugin install superpowers@superpowers-marketplace
 ```
 
 ### Verify Installation
 
+Check that commands appear:
+
 ```bash
-# Check that commands appear
 /help
+```
 
+```
 # Should see:
 # /superpowers:brainstorm - Interactive design refinement
 # /superpowers:write-plan - Create implementation plan