Add heading for response formats for pull requests
Explain and deprecate the merge_commit_sha

The merge_commit_sha is often misunderstood. It is internal GitHub
plumbing that just happens to be part of the public API. Explain it
(to reduce the confusion for the current version of the API), and
announce plans to remove it from the next version of the API.
Link merge_commit_sha explanation to deprecation
Improve "mergability" docs

Thanks to @gjtorikian for the suggestion.
Clarify supported media types for Contents API
Merge pull request #244 from github/clarify-supported-media-types-for-repo-contents-api

Clarify supported media types for Repository Contents API
Fix URLs in symlink example response
/repos/:owner/:repo/hooks/:id/tests is `push` only

Clarify that /repos/:owner/:repo/hooks/:id/tests will only trigger a callback for hooks that are subscribed to `push` events while listeners for non-`push` events will receive nothing.
Merge pull request #246 from dsheets/patch-2

/repos/:owner/:repo/hooks/:id/tests is `push` only
Required UA post
Merge pull request #236 from github/deprecate-merge-commit-sha

Explain & deprecate merge_commit_sha
Announce deprecation of merge_commit_sha
Fixing url in tree object to correct /repos url
Merge pull request #247 from runemadsen/patch-1

Fixing url in tree object to correct /repos url
Document file sizes supported by Repo Contents API
Document submodule handling in Repo Contents API
Announce plans to fix `type` value for submodules

Prior to 2013-04-30, you could not GET a submodule via the Repository
Contents API. (You could see the submodule listed when you GET the
directory that the submodule resides it, but you could not GET just the
submodule.) On 2013-04-30, we added the ability to GET a submodule.
(See 81c7e58.) When you GET a submodule, the "type" is returned as
"submodule".

Prior to 2013-04-30, when you listed the contents of a directory using
the Repository Contents API, the output included submodules, but it
returned the "type" as "file". The type *should* be "submodule". We are
leaving this behavior as is for v3, so as not to break any client code
that might depend on this behavior. In the next major version of the
API, the type will be returned as "submodule".
Commit statuses now work for all branches and tags
Blog post for branch/tag statuses
Fix typos and change space+newline -> newline