Add a setup gotchas section to arch.md.

[dnwpark]

[dnwpark]

My setup issue was because my environment was messed up.

[1st1]

Add a note about your warnings -- add a section "gotchas" or FAQ, copy/paste your warnings, add the recommendation + explanation (pyx files were changed, run make clean && make to force-recompile all cython code)

[dnwpark]

@1st1 I couldn't reproduce some of the earlier errors, but hopefully the next person to encounter them will take better notes.

[1st1]

@1st1 I couldn't reproduce some of the earlier errors

just copy paste from slack, errors are always the same and the cause is well-known