Add Creation With Upload extension (#88)

* Add first draft of the Creation With Chunk functionality

* Improve wording of Create With Upload extension

* Move Creation With Upload into own section

* Rename Creation With Upload value in Tus-Extension

* Apply suggestions from @kvz and @AJvanLoon

protocol.md

@@ -363,6 +363,59 @@ resource. This URL MAY be absolute or relative.
 
 The Client MUST perform the actual upload using the core protocol.
 
+### Creation With Upload
+
+The Client MAY include parts of the upload in the initial Creation request
+using the Creation With Upload extension.
+
+If the Server supports this extension, it MUST advertise this by including
+`creation-with-upload` in the `Tus-Extension` header. Furthermore, this extension
+depends directly on the Creation extension. Therefore, if the Server does not
+offer the Creation extension, it MUST NOT offer the Creation With Upload
+extension either.
+
+The Client MAY include either the entirety or a chunk of the upload data in the body of
+the `POST` request. In this case, similar rules as for the `PATCH` request and
+response apply. The Client MUST include the
+`Content-Type: application/offset+octet-stream` header. The Server SHOULD accept
+as many bytes as possible and MUST include the `Upload-Offset` header in the
+response and MUST set its value to the offset of the upload after applying the
+accepted bytes.
+
+If the Client wants to use this extension, the Client SHOULD verify that it is
+supported by the Server before sending the `POST` request.
+In addition, the Client SHOULD include the `Expect: 100-continue` header in
+the request to receive early feedback from the Server on whether it will accept
+the creation request, before attempting to transfer the first chunk.
+
+#### Example
+
+A non-empty `POST` request is used to create a new upload resource. The
+`Upload-Offset` header in the response indicates how much data has been accepted.
+
+**Request:**
+
+```
+POST /files HTTP/1.1
+Host: tus.example.org
+Content-Length: 5
+Upload-Length: 100
+Tus-Resumable: 1.0.0
+Content-Type: application/offset+octet-stream
+
+hello
+```
+
+**Response:**
+
+```
+HTTP/1.1 201 Created
+Location: https://tus.example.org/files/24e533e02ec3bc40c387f1a0e460e216
+Tus-Resumable: 1.0.0
+Upload-Offset: 5
+```
+
+
 ### Expiration
 
 The Server MAY remove unfinished uploads once they expire. In order to indicate