Added note on grobid concurrency configuration to README.

[elshimone]

Fixes #50

[davidmezzetti]

Sorry to nitpick here but could we make this a bit more concise? In my experience if we have a lot of these type of edge case messages over time, it gets hard to read.

Maybe something like this:

Note: Depending on the number of CPUs in your system, the GROBID engine pool may be exhausted when parsing PDFs, resulting in a 503 error. This can be fixed by increasing the concurrency and/or poolMaxWait setting in the GROBID configuration file.

If you're strapped for time, I can handle myself. You've already done a bunch to help and it's been greatly appreciated.

[davidmezzetti]

Never mind, I made the minor edit. Thank you for the additions to paperetl and paperai!