Provide basic documentation about the script

[bluepuma77]

You want us to (blindly) execute a shell script, it would be great to provide some basic info in the README:

1. Where is the script installed?
2. What for and how is the email used?
3. Why a cron job is installed and what is it doing?

I just want to create certs with a DNS provider. Probably I can set email with other credentials and don't need any cron job.

[user8446]

The install is actually in acme.sh, all this does is download it and enter your email address.

AI definition:

1. It defines a function _exists that checks if a given command is available on the system.

2. It sets the BRANCH variable to "master" if it's not already defined.

3. It extracts the _email parameter from the command-line arguments, which is expected to be in the format "email=my@example.com".

4. It constructs the URL "https://raw.githubusercontent.com/acmesh-official/acme.sh/$BRANCH/acme.sh" to download the acme.sh script.

5. It determines the appropriate method to download the script, either using curl or wget, and stores the command in the _get variable.

6. Finally, it downloads the acme.sh script and runs it with the --install-online option, passing the _email parameter and any additional arguments.

If the installation fails, the script prints an error message and provides a link to the installation instructions for users in China.