Define that DownloadFile content hash method is SHA256 (#68)

Resolves #67

Without specifying the hashing method it was useless for the purposes it claims to exist for
>The hash of the file content. Can be used by the Agent to verify that the file was downloaded correctly.

You can't verify that the hash matches the content if you don't know the hashing method.

Now it is precisely defined to be SHA256 of the content.

specification.md

@@ -1923,7 +1923,7 @@ There are 3 levels of hashes:
 
 <h4 id="file-hash">File Hash</h4>
 
-The hash of the addon file content. This is stored in the [hash](#hash) field in
+The hash of the addon file content. This is stored in the [content_hash](#content_hash) field in
 the [DownloadableFile](#downloadablefile-message) message. This value SHOULD be
 used by the Agent to determine if the particular file it has is different on the
 Server and needs to be re-downloaded.
@@ -2046,9 +2046,9 @@ The URL from which the file can be downloaded using HTTP GET request. The server
 at the specified URL SHOULD support range requests to allow for resuming
 downloads.
 
-<h4 id="content_hash">content_hash</h4>
+#### content_hash
 
-The hash of the file content. Can be used by the Agent to verify that the file
+The SHA256 hash of the file content. Can be used by the Agent to verify that the file
 was downloaded correctly.
 
 #### signature