Add executable permissions step to installation instructions

[samsternberg]

Problem

After running npm run build, the compiled dist/index.js file does not have executable permissions set. This causes a "permission denied" error when trying to run the sky command after global installation with npm i -g ..

Error Example

$ sky
zsh: permission denied: sky

Solution

Add a chmod +x dist/index.js step to the installation instructions in the README, between the build step and the global installation step.

Expected Outcome

Users following the installation instructions will not encounter permission errors when running the sky command.