README doesn't mention this action requires bash on PATH (Windows)

action.yml hardcodes \`shell: bash\` for every step. This works transparently on GitHub-hosted \`windows-latest\`, which ships Git Bash on PATH by default, but the README doesn't mention this dependency anywhere.

On a self-hosted Windows runner without Git Bash on PATH (e.g. a bare Windows Server AMI with only Git for Windows installed via its \`cmd\` directory, not \`bin\`), the action fails immediately with a bare:

\`\`\`
##[error]bash: command not found
\`\`\`

with no indication of why, since nothing in the README or action output mentions bash is required at all. Took a while to trace back to this, since the assumption is invisible on \`windows-latest\` and only surfaces on custom Windows runners.

Would be a quick, useful addition to the README: mention that self-hosted Windows runners need \`bash\` (e.g. Git Bash) available on PATH for this action to work.

Read README.md alongside action.yml to confirm the action's shell requirement and where runtime prerequisites are documented. Update the README to state that self-hosted Windows runners need bash, such as Git Bash, available on PATH; done when this dependency and the relevant failure context are clear to users.

Written by the indexing model from the issue text.