Add install from source instructions to README

[jdetter]

Description of Changes

Please describe your change, mention any related tickets, and so on here.

Fixes #2420

Rendered view: https://github.com/clockworklabs/SpacetimeDB/blob/jdetter/docs/2420/README.md

API and ABI breaking changes

If this is an API or ABI breaking change, please apply the
corresponding GitHub label.

NA

Expected complexity level and risk

How complicated do you think these changes are? Grade on a scale from 1 to 5,
where 1 is a trivial change, and 5 is a deep-reaching and complex change.

This complexity rating applies not only to the complexity apparent in the diff,
but also to its interactions with existing and future code.

If you answered more than a 2, explain what is complex about the PR,
and what other components it interacts with in potentially concerning ways.

0 - just a docs update

Testing

Describe any testing you've done, and any testing you'd like your reviewers to do,
so that you're confident that all the changes work as expected!

• I have ran all install instructions and verified that they're working correctly.
• Linux install instructions
• macOS install instructions - not tested but the Linux install instructions are identical and it worked on Linux
• Windows install instructions

[joshua-spacetime]

Are you able to add the same message about adding ~/.local/bin to your path on macos that the install script does? Technically this would only matter for someone who has never used the official install script on macos.

[joshua-spacetime]

Approving, if you could just update the macos instructions before merging.

[jdetter]

I believe I've addressed everything here 👍