Add doc for unit tests

Docs/UnitTests.md

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+Rainmeter uses the [Visual Studio Native Unit Testing framework](http://msdn.microsoft.com/en-us/library/hh598953.aspx) for unit testing.
+
+
+### General notes
+
+All unit test files should have the `<ExcludedFromBuild>$(ExcludeTests)</ExcludedFromBuild>` property set. The easiest way to ensure this is to manually edit the `ClCompile` entry of the project .vcxproj file. Make sure it looks something like this:
+
+        <ClCompile Include="ConfigParser_Test.cpp">
+          <ExcludedFromBuild>$(ExcludeTests)</ExcludedFromBuild>
+        </ClCompile>
+
+When building within Visual Studio, the macro `$(ExcludeTests)` evaluates to `false` (i.e. the test files are **not** excluded). When building using Build.bat, it evaluates to `true`, and, consequently, the test files are excluded from the build. This is done in order to both reduce the actual build size and remove any kind of dependency on *Microsoft.VisualStudio.TestTools.CppUnitTestFramework.dll* (see below).
+
+
+### Project type specific notes
+
+* Static library projects:
+
+    Unit testing static library project requires the use of a separate DLL project. The static library project contains the actual code while the DLL project references the static library and includes the testing code.
+
+    See `Common\Common.vcxproj` and `Common\Common_Test.vcxproj` for an example.
+
+* Dynamic library projects:
+
+    A unit tested dynamic library project should contain both the actual code and the testing code.
+
+    In addition, the `$(DelayLoadTestDLL)` macro should be included in the linker *Delay Loaded Dlls* option. The VS unit testing framework introduces a dependency on *Microsoft.VisualStudio.TestTools.CppUnitTestFramework.dll* for dynamic library projects, which means that omitting the `$(DelayLoadTestDLL)` macro will result in the dynamic library failing to load.