Very basic unit test documentation

Summary: This needs work but is better than nothing.
Test Plan: Generated and read documentation, clicked links.
Reviewed By: edward
Reviewers: edward
CC: aran, edward
Differential Revision: 765

src/docs/developer/unit_tests.diviner

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+@title Writing Unit Tests
+@group developer
+
+Simple guide to libphutil, Arcanist and Phabricator unit tests.
+
+= Overview =
+
+libphutil, Arcanist and Phabricator provide and use a simple unit test
+framework. This document is aimed at project contributors and describes how to
+use it to add and run tests in these projects or other libphutil libraries.
+
+In the general case, you can integrate ##arc## with a custom unit test engine
+(like PHPUnit or any other unit testing library) to run tests in other projects.
+See @{article:Arcanist User Guide: Customizing Lint, Unit Tests and Workflows}
+for information on customizing engines.
+
+= Adding Tests =
+
+To add new tests to a libphutil, Arcanist or Phabricator module:
+
+  - Create a ##__tests__/## directory in the module if it doesn't exist yet.
+  - Add classes to the ##__tests__/## directory which extend from
+    @{class:PhabricatorTestCase} (in Phabricator) or
+    @{class@arcanist:ArcanistPhutilTestCase} (elsewhere).
+  - Run ##arc liberate## on the library root so your classes are loadable.
+
+= Running Tests =
+
+Once you've added test classes, you can run them with:
+
+  - ##arc unit path/to/module/##, to explicitly run module tests.
+  - ##arc unit##, to run tests for all modules affected by changes in the
+    working copy.
+  - ##arc diff## will also run ##arc unit## for you.