Docs: updated tutorial - changes to contract

docs/source/tutorial.rst

@@ -148,18 +148,55 @@ And we can see that it is overriding the original version:
 Submitting contract change
 --------------------------
 
+You can make changes to the contract in cases described in the 4th part of Article 36 of the Law "On the Public Procurement". 
+
+All changes are processed by the endpoint `/contracts/{id}/changes`.
+
+`Change` is a submit of new `Change` object to the `changes` container. 
+
+Required field: ``rationale``.
+
 .. include:: tutorial/add-contract-change.http
    :code:
 
+You can view the `change`:
+
+.. :code:
+
+`Change` can be modified while it is in the ``pending`` status:
+
 .. include:: tutorial/patch-contract-change.http
    :code:
 
+Document can be added only while `change` is in the ``pending`` status.
+
+Document has to be added in two stages:
+
+1) add document
+
 .. include:: tutorial/add-contract-change-document.http
    :code:
 
+
+2) add connection to the `change`:
+
+      ``"documentOf": "change"``
+
+      ``"relatedItem": "{change.id}"``
+
+.. :code:
+
+`Change` has to be applied by switching to the ``active`` status. After this `change` can't be modified anymore.
+
 .. include:: tutorial/apply-contract-change.http
    :code:
 
+You can view all changes:
+
+.. :code:
+
+All changes are present on the contract view.
+
 Completing contract
 -------------------