Add RUSTDOCFLAGS to README

[sharksforarms]

RUSTDOCFLAGS is used to pass arguments to doc tests, RUSTFLAGS is unfortunately not passed resulting in the following errors:

failures:

---- src/lib.rs -  (line 11) stdout ----
error: the crate `tester` is compiled with the panic strategy `abort` which is incompatible with this crate's strategy of `unwind`

error: aborting due to 4 previous errors

Couldn't compile the test.

failures:
    src/lib.rs -  (line 11)

Found RUSTDOCFLAGS here: rust-lang/cargo#6650

Upstream tracking issue: rust-lang/rust#43031

If there's a way to reference $RUSTFLAGS in the yaml somehow, let me know and I can update it! For now I just pasted RUSTFLAGS