[Add]: CustomValidationTests.test_partial_update

This test is to make sure that validate_<attrname> is not called when
partial=True and <attrname> is not found in .data.
Be sure to import UserSerializer
fix writing into foreign key with non-null source
fix ticket link in test docstring
Merge pull request #1157 from dpetzel/tutorial_authentication

Be sure to import UserSerializer
Add missing commas in relations.md
Merge pull request #1171 from alazaro/patch-1

Add missing commas in relations.md
[Fix]: Error with partial=True and validate_<name>

The error occurs when serializer is set with partial=True and a
field-level validation is defined on a field, for which there's no
corresponding update value in .data
Added name of file to edit

So reader doesn't have to remember, or check through all the files to find where this code fragment was, mention the file name when it is relevant.
Mention name of file to edit

To reduce unnecessary cognitive load of the learner, name the file they are putting this code in.
Using the filenames where relevant

Sometimes it's hard to tell which file the code is intended to go in. Now it spells it out.
Allow to customize description so that markup can be accepted if needed.
Merge pull request #1180 from kevin-brown/pull_1164

Put template block around the description
Fix typo YAMLRendererTests
Fix decimal support with YAMLRenderer
Merge pull request #1181 from badale/master

Fixed #1179: RepresenterError with DecimalField and YAMLRenderer
Added @badale, for work on #1179.  Thanks!
Fix a docstring to reflect what the method does
Merge pull request #1172 from brutasse/fix/return-object

Return object in save_object
Merge pull request #1112 from tamakisquare/issue-1111

Test case and fix for issue 1111