message: Explain why we are not using 'cache: cargo' with Travis
diff: @@ -6,6 +6,10 @@ dist: trusty
  git:
  depth: 1
  +# Using 'cache: cargo' to cache target/ and all of $HOME/.cargo/
  +# doesn't work well: the cache is large and it takes several minutes
  +# to move it to and from S3. So instead we only cache the mdbook
  +# binary.
  cache:
  directories:
  - $HOME/.cargo/bin/
  