Added Usage instructions

Added Usage instructions

Fixes #165

R=jmesserly@google.com, leafp@google.com

Committed: https://github.com/dart-lang/dev_compiler/commit/c37fea0d51dd34d3aa5283ac3a3caffc5b65c3c8

[vsm, 2015-05-28 17:35:10]

vsm@google.com changed reviewers:
+ dgrove@google.com, jmesserly@google.com, leafp@google.com

[Leaf, 2015-05-28 18:27:25]

lgtm with typo fix.

https://codereview.chromium.org/1157093004/diff/1/USAGE.md
File USAGE.md (right):

https://codereview.chromium.org/1157093004/diff/1/USAGE.md#newcode54
USAGE.md:54: DDC output may be testing most easily in [Chrome
s/testing/tested/

[vsm, 2015-05-28 18:28:52]

Fix typo

[vsm, 2015-05-28 18:29:27]

https://codereview.chromium.org/1157093004/diff/1/USAGE.md
File USAGE.md (right):

https://codereview.chromium.org/1157093004/diff/1/USAGE.md#newcode54
USAGE.md:54: DDC output may be testing most easily in [Chrome
On 2015/05/28 18:27:25, Leaf wrote:
> s/testing/tested/

Done.

[Jennifer Messerly, 2015-05-28 19:26:50]

lgtm. I guess you already fixed one of them :)

https://codereview.chromium.org/1157093004/diff/1/USAGE.md
File USAGE.md (right):

https://codereview.chromium.org/1157093004/diff/1/USAGE.md#newcode39
USAGE.md:39: will display a severe error.  Modifying ```42``` to ```'42'``` will
can use single backtick instead of triple for inline code:

`42`

instead of

```42```

https://codereview.chromium.org/1157093004/diff/1/USAGE.md#newcode54
USAGE.md:54: DDC output may be testing most easily in [Chrome
s/testing/tested/

[vsm, 2015-05-29 13:02:52]

https://codereview.chromium.org/1157093004/diff/1/USAGE.md
File USAGE.md (right):

https://codereview.chromium.org/1157093004/diff/1/USAGE.md#newcode39
USAGE.md:39: will display a severe error.  Modifying ```42``` to ```'42'``` will
On 2015/05/28 19:26:50, John Messerly wrote:
> can use single backtick instead of triple for inline code:
> 
> `42`
> 
> instead of
> 
> ```42```

Done.

[vsm, 2015-05-29 13:04:46]

Committed patchset #3 (id:40001) manually as
c37fea0d51dd34d3aa5283ac3a3caffc5b65c3c8 (presubmit successful).