update README to set RUSTDOCFLAGS too

[davepacheco]

See #213. I somehow missed that if you try to work around this by setting RUSTFLAGS at build time but not RUSTDOCFLAGS, then the oximeter doc-tests fail as well:

test result: ok. 0 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out; finished in 0.00s

   Doc-tests oximeter

running 8 tests
test src/traits.rs - traits::Metric (line 117) ... ok
test src/traits.rs - traits::Target (line 50) ... ok
test src/traits.rs - traits::Metric (line 124) ... ok
test src/traits.rs - traits::Target (line 24) ... FAILED
test src/traits.rs - traits::Producer (line 202) ... FAILED
test src/histogram.rs - histogram::Histogram<T>::with_bins (line 246) ... FAILED
test src/traits.rs - traits::Metric (line 99) ... FAILED
test src/histogram.rs - histogram::Histogram (line 191) ... FAILED

failures:

---- src/traits.rs - traits::Target (line 24) stdout ----
Test executable failed (terminated by signal).

stderr:
ld.so.1: rust_out: fatal: libpq.so.5: open failed: No such file or directory


---- src/traits.rs - traits::Producer (line 202) stdout ----
Test executable failed (terminated by signal).

stderr:
ld.so.1: rust_out: fatal: libpq.so.5: open failed: No such file or directory


---- src/histogram.rs - histogram::Histogram<T>::with_bins (line 246) stdout ----
Test executable failed (terminated by signal).

stderr:
ld.so.1: rust_out: fatal: libpq.so.5: open failed: No such file or directory


---- src/traits.rs - traits::Metric (line 99) stdout ----
Test executable failed (terminated by signal).

stderr:
ld.so.1: rust_out: fatal: libpq.so.5: open failed: No such file or directory


---- src/histogram.rs - histogram::Histogram (line 191) stdout ----
Test executable failed (terminated by signal).

stderr:
ld.so.1: rust_out: fatal: libpq.so.5: open failed: No such file or directory



failures:
    src/histogram.rs - histogram::Histogram (line 191)
    src/histogram.rs - histogram::Histogram<T>::with_bins (line 246)
    src/traits.rs - traits::Metric (line 99)
    src/traits.rs - traits::Producer (line 202)
    src/traits.rs - traits::Target (line 24)

test result: FAILED. 3 passed; 5 failed; 0 ignored; 0 measured; 0 filtered out; finished in 14.66s

error: test failed, to rerun pass '--doc'