docs: add mise installation instructions (#526)

Add mise (polyglot runtime manager) as an installation option using the
ubi backend to install from GitHub releases.

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Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

docs/INSTALLING.md

@@ -17,6 +17,18 @@ brew install bd
 - ✅ No need to install Go
 - ✅ Handles PATH setup automatically
 
+### mise (polyglot runtime manager)
+
+```bash
+mise use -g ubi:steveyegge/beads[exe=bd]
+```
+
+**Why mise?**
+- ✅ Single tool for managing many development tools
+- ✅ Version pinning per project via `mise.toml`
+- ✅ No need to install Go
+- ✅ Works across macOS, Linux, and Windows
+
 ### Quick Install Script (All Platforms)
 
 ```bash