Clarify talos:bootstrap setup instructions

[mkoval]

I ran into two issues while running to bootstrap my Talos cluster:

1. The cluster was left in a partially-initialized, non-functional state after running the command without the required environment variables were not set (see also: Clarify direnv setup instructions #1437).
2. After fixing that issue, I assumed that all of the permission errors spam indicated that something was still wrong - so I Ctrl + Ced the process after a few minutes.

This PR adds a note to the README that the talos:bootstrap command (1) may take a while, (2) outputs a bunch of error messages, and (3) may require nuking the cluster if interrupted.

[onedr0p]

Thanks!