added note for 'eh_personality' error

[U007D]

trafficstars

If one happens to have a standard profile set to unwinding panics, a language item required, but not found: 'eh_personality' will result. This PR adds a brief note explaining how to resolve the error.

[austinglaser]

I'm in favor of adding this language -- I ran into this exact issue when working through the book.

[bors[bot]]

Build failed:

• continuous-integration/travis-ci/push

[jamesmunns]

Note: will probably need to be rebased on https://github.com/rust-embedded/embedonomicon/pull/66