Add note about CARGO_WORKSPACE_DIR must be set if CARGO_TARGET_DIR is

[faern]

Companion PR with esp-rs/esp-idf-sys#221. This caveat makes me waste a lot of time every time I get into ESP programming. I don't know how common it is in general to set CARGO_TARGET_DIR, but I have it set to ~/.cargo/target all the time. And embuild does not like this.

[faern]

Closing due to staleness/lack of interest from upstream.

[ivmarkov]

Hey - sorry for not following up here.

I noticed a couple of months ago, that my subscription to a few esp-idf-* projects was of "only if I'm mentioned" type, rather than for everything.
This might have not been the case with esp-idf-template, but surely was the case with esp-idf-sys. Here, I might just have forgotten about the PR. :(

Regardless - if you re-open it, I'll merge. No reason not to, looking at the change.
Next time, maybe you just ping somebody with committer privilege. I tend not to be very organized and forget to follow up on stuff sometimes. Sorry again about that!

[faern]

Reopened! However, there are conflicts. And I'm not on the correct computer to fix that right now. I also don't know if this is still an issue or not! Have not tried it for a long time now. But thanks for replying!

[ivmarkov]

I'll wait until you find time to resolve the conflicts.

As for whether it is still an issue - I don't recall anybody changing the piece of code in esp-idf-sys/build which is trying to locate the crate root, so I would bet it is still pretty much valid.