Update README with 'uv' installation instructions

Added instructions for installing 'uv' tool.

Author: JanuszNowak

README.md

@@ -53,6 +53,11 @@ Install once and use everywhere:
 ```bash
 uv tool install specify-cli --from git+https://github.com/github/spec-kit.git
 ```
+If missing uv
+
+```bash
+irm https://astral.sh/uv/install.ps1 | iex
+```
 
 Then use the tool directly: