Add README.md (#8)

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README.md

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+# asdf-cairo-coverage
+
+[cairo-coverage] plugin for the [asdf] version manager.
+
+## Install
+
+This plugin needs `bash`, `curl`, `tar` and other generic POSIX utilities.
+Everything should be included by default on your system.
+
+```shell
+asdf plugin add cairo-coverage https://github.com/software-mansion/asdf-cairo-coverage.git
+```
+
+## Use
+
+Show all installable versions:
+
+```shell
+asdf list-all cairo-coverage
+```
+
+Install latest version:
+
+```shell
+asdf install cairo-coverage latest
+```
+
+Install specific version:
+
+```shell
+asdf install cairo-coverage 0.2.0
+```
+
+Set a version globally (in your `~/.tool-versions` file):
+
+```shell
+asdf global cairo-coverage latest
+```
+
+Now cairo-coverage commands are available:
+
+```shell
+cairo-coverage --version
+```
+
+Check [asdf](https://github.com/asdf-vm/asdf) readme for more instructions on how to install & manage versions.
+
+[asdf]: https://asdf-vm.com
+
+[cairo-coverage]: https://github.com/software-mansion/cairo-coverage