Add usage example to README

[davidpdrsn]

And have it checked using https://github.com/budziq/rust-skeptic

Checking code in readmes might eventually become part of Cargo but currently isn't.

[davidpdrsn]

rust-skeptic seems to not actually be working.

I tried setting this up, but was getting errors like

running 1 test
error[E0464]: multiple matching crates for `robin`
 --> /var/folders/bj/lr1dkdx94psb8lrybqt66kkc0000gn/T/rust-skeptic.L2iwu33PH8JQ/test.rs:3:1
  |
3 | extern crate robin;
  | ^^^^^^^^^^^^^^^^^^^
  |
  = note: candidates:
          crate `robin`: /Users/davidpdrsn/source/projects/robin/robin/target/debug/deps/librobin-32dcaf7269eef9b4.rlib
          crate `robin`: /Users/davidpdrsn/source/projects/robin/robin/target/debug/deps/librobin-7f4c8cbd22e1046d.rlib
          crate `robin`: /Users/davidpdrsn/source/projects/robin/robin/target/debug/deps/librobin-95fb9e0bfcc37e09.rlib
          crate `robin`: /Users/davidpdrsn/source/projects/robin/robin/target/debug/deps/librobin-a66cca6534d28ef5.rlib
          crate `robin`: /Users/davidpdrsn/source/projects/robin/robin/target/debug/deps/librobin-1aaf7c136f76e1c7.rlib
          crate `robin`: /Users/davidpdrsn/source/projects/robin/robin/target/debug/deps/librobin-c7020322da804ddc.rlib

error[E0463]: can't find crate for `robin`
 --> /var/folders/bj/lr1dkdx94psb8lrybqt66kkc0000gn/T/rust-skeptic.L2iwu33PH8JQ/test.rs:3:1
  |
3 | extern crate robin;
  | ^^^^^^^^^^^^^^^^^^^ can't find crate

error: aborting due to 2 previous errors

test readme_sect_installation_line_26 ... FAILED

failures:

---- readme_sect_installation_line_26 stdout ----
        thread 'readme_sect_installation_line_26' panicked at 'Command failed:
"rustc" "/var/folders/bj/lr1dkdx94psb8lrybqt66kkc0000gn/T/rust-skeptic.L2iwu33PH8JQ/test.rs" "--verbose" "--crate-type=bin" "-L" "/Users/davidpdrsn/source/projects/robin/robin/target/d
ebug" "-L" "/Users/davidpdrsn/source/projects/robin/robin/target/debug/deps" "--target" "x86_64-apple-darwin" "--extern" "num_cpus=/Users/davidpdrsn/source/projects/robin/robin/target/
debug/deps/libnum_cpus-c0dba8152ea7dbb1.rlib" "--extern" "skeptic=/Users/davidpdrsn/source/projects/robin/robin/target/debug/deps/libskeptic-0de2885e4aaf24b5.rlib" "-o" "/var/folders/b
j/lr1dkdx94psb8lrybqt66kkc0000gn/T/rust-skeptic.L2iwu33PH8JQ/out.exe"', /Users/davidpdrsn/.cargo/registry/src/github.com-1ecc6299db9ec823/skeptic-0.13.2/lib.rs:818:13
note: Run with `RUST_BACKTRACE=1` for a backtrace.

I think we should wait with this until the API has stabilised, but currently it is too much influx that I'd be worried about having out of date code in the readme.