Code of conduct

Common rules for contributing to acme.sh

1. The file shebang must be sh not bash

acme.sh is a `unix shell` script,  not just a `bash` script.

If you want to contribute your script, the shebang must be: 
```sh
#!/usr/bin/env sh
```
After the installation, acme.sh could change the shebang to bash to get better performance if you have bash on your machine.

Of course, if you just use it on your own, it can be any valid shebang on your machine. It could be `sh` or `bash`, it's up to you.