Improve Developer Onboarding on README

[fenn-cs]

The onboarding process for new contributors on the read me can be improved, we can make a few things more accessible by adding them directly on the readme and linking to detailed pages more complex tasks.

Additionally, new contributors need to dig into the docs directory to find the correct installation files due to broken links in the install section.

Broken Links

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• ​https://github.com/tahoe-lafs/tahoe-lafs/blob/master/docs/INSTALL.rst
• ​https://github.com/tahoe-lafs/tahoe-lafs/blob/master/docs/windows.rst
• ​https://github.com/tahoe-lafs/tahoe-lafs/blob/master/docs/OS-X.rst

[exarkun]

Hello. Thanks for filing the ticket. Can you edit it to more precisely define the problem?

For example, is the problem just that there are three broken links? If so then instead of saying the document can be "improved" and made "more accessible", just say what the broken links are. If there are other specific problems, describe those. Also, consider creating more tickets if there are more problems - not bunching a lot of separate things into one ticket.

[maylee]

Collect broken links into this ticket:

​https://tahoe-lafs.org/trac/tahoe-lafs/ticket/3749