[Issue 17998] New: Document Options for install.sh

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=17998

          Issue ID: 17998
           Summary: Document Options for install.sh
           Product: D
           Version: D2
          Hardware: x86
                OS: Windows
            Status: NEW
          Severity: enhancement
          Priority: P1
         Component: dlang.org
          Assignee: nobody at puremagic.com
          Reporter: john.michael.hall at gmail.com

The D install script install.sh is mentioned on the website a few times, but
all of its options are not described. I was surprised to learn from [1] that
you could use it to install ldc.

1] https://github.com/ldc-developers/ldc/pull/2421#pullrequestreview-77997374

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