Add documentation for additional --tag arguments

[dgannon991]

Summary

There are three commands which now accept the --tag argument:

• pack buildpack package
  

• pack extension package
  

• pack builder create
  

These changes should be reflected

What questions should this documentation answer?

• How to use the --tag argument

Who is the target audience for this documentation?

• App Developer
• Buildpack Author
• Operator
• Other:

Additional Information

The tag feature is being tracked in buildpacks/pack#2367