Docs: add bootstrap note

[luong-komorebi]

This note is important because there are certain caveats when bootstrapping fisher:

1. User without this instruction might run into a loop. #644
2. User who follows the standard installation process and trying to couple that into a bootstrap script will run into this issue #670 where the whole fish_plugins is overwritten

I borrowed the wording from this commit dadcc6c

[jorgebucaran]

Hey, @luong-komorebi, thanks, but I'll pass. We don't promote bootstrapping Fisher anywhere like that, precisely because of all the caveats, so while this comment is useful, we don't need it at this time.