Document how initial boot.jar can be downloaded with proxy

[martinklepsch]

The boot.jar file that is used for bootstrapping the environment is currently downloaded from Github and specifying a proxy for that operation is non-obvious. A solution pointed out by @micha in https://github.com/boot-clj/boot-bin/issues/2 has been tested by multiple people and seems to work:

BOOT_JVM_OPTIONS='-Dhttps.proxyHost=1.2.3.4' boot ...

This should be documented in a proper wiki page. Maybe a page For Proxy Users could describe this as well has usage of the :proxy boot-env key.

related: https://github.com/boot-clj/boot/issues/604

[unedited-despair]

Any update?

EDIT: Ah, I see. https://github.com/boot-clj/boot/wiki/Configuring-Boot

EDIT: and: https://github.com/boot-clj/boot/wiki/JVM-Options