Make zsh plugin usable straight from the repo

Author: aserrallerios

etc/README.md

@@ -0,0 +1,28 @@
+# Installation instructions
+
+## zsh
+
+Create a new folder for completions:
+
+```sh
+mkdir -p ~/.zsh/completions
+```
+
+Copy the file `/etc/_sdk` from the location where `sdk` is installed to the folder `~/.zsh/completions/`:
+
+```sh
+cp /path/to/etc/sdk.zsh_completion ~/.zsh/completions/
+```
+
+Then add the following lines to your `.zshrc` file:
+
+```sh
+fpath=(~/.zsh/completions $fpath)
+autoload -U compinit && compinit
+```
+
+### Using antigen
+
+```sh
+antigen bundle sdkman/sdkman-cli zsh
+```