[krbdev.mit.edu #8511] Update references to build instructions in README

[Karl-Philipp Richter via RT]

Hi,
`README` mentions build instructions at `doc/pdf/build.pdf`, but
according to `find . -name '*.pdf'` the only PDF is
`./src/lib/crypto/crypto_tests/test/katmct.pdf` which is unrelated.

experienced with c69d0b7

-Kalle