Add installation instructions to README

Include cabal install command and PATH setup guidance for installing
docster as a system-wide CLI tool.

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

README.md

@@ -9,6 +9,16 @@ A self-contained CLI tool that converts Markdown with Mermaid diagrams into PDF
 
 Mermaid code blocks (```mermaid) will be rendered to SVG and embedded.
 
+## Installation
+
+### Build and install to PATH
+
+    cabal install --installdir=$HOME/.local/bin
+
+Make sure `~/.local/bin` is in your PATH. Add to your shell config if needed:
+
+    export PATH="$HOME/.local/bin:$PATH"
+
 ## Requirements
 
 - GHC + Cabal (via ghcup)