Add mise as a recommended installation method

Abre README.md y localiza la sección Installation y sus métodos existentes. Añade al principio la sección solicitada «via mise (recommended)» con los comandos globales y de proyecto documentados. Después, verifica que Markdown se renderice correctamente y que las instrucciones coincidan con el issue.

Document mise as an installation method

Proposal

Add mise as the recommended way to install the OpenFeature CLI.

mise is a developer tool version manager (>27k GitHub stars) that provides deterministic, reproducible toolchains pinned per-project via a config file checked into version control. Using the github: backend, mise can install the CLI directly from GitHub Releases without requiring a registry entry:

# Install globally
mise use --global "github:open-feature/cli"

# Or install in the current project
mise use "github:open-feature/cli"

This would also allow teams to pin the CLI version in their project's mise.toml:

[tools]
"github:open-feature/cli" = "0.4.0"

The github: backend automatically downloads the correct pre-built binary from GitHub Releases based on the user's OS and architecture — no compilation required.

Why mise?

• Reproducible environments: Every developer on a team gets the exact same CLI version, managed declaratively in a config file.
• No manual upgrades: mise upgrade "github:open-feature/cli" handles it.
• Multi-tool management: Teams already using mise for Go, Node, Python, etc. get the OpenFeature CLI in the same workflow.
• Cross-platform: Works on macOS, Linux, and Windows.
• Wide adoption: mise is one of the most popular dev tool managers in the ecosystem, used by a large and growing community.

Implementation

This requires no upstream changes — no registry PR, no build changes. The existing GoReleaser setup already produces the release artifacts that mise's github: backend expects.

The only change needed is adding a ### via mise (recommended) section at the top of the Installation docs in README.md, before the existing methods:

### via mise (recommended)

The recommended way to install the OpenFeature CLI is with [mise](https://mise.jdx.dev), a developer tool version manager:

```bash
# Install globally
mise use --global "github:open-feature/cli"

# Or install in the current project
mise use "github:open-feature/cli"
```

How it works

The github: backend auto-detects the correct archive from GitHub Releases using the asset naming pattern (openfeature_Darwin_x86_64.tar.gz, etc.), which aligns with the existing GoReleaser name_template. It downloads the archive, extracts the binary, and places it on the user's PATH.

Future improvements

Once the CLI gains more adoption, we could register it in the mise registry, which would allow a shorter install command:

mise use openfeature