In README.md reference to LMIC-v3.3.0.pdf needs update

[lnlp]

What document is involved?

README.md

What changes are needed?

The following needs to be updated for 4.0:

PDF/Word Documentation
The doc directory contains LMIC-v3.3.0.pdf, which documents the library APIs and use.

[terrillmoore]

Thanks!

[omrisarig13]

The same issue exists in the README.md under the doc directory (it references LMIC-v3.3.0.pdf and LMIC-v3.3.0-redline.pdf instead of LMIC-v4.0.0.pdf and LMIC-v4.0.0-redline.pdf)