d Sketch out docs for possible Custom Namer feature

This is to build on #130

Co-Authored-By: Llewellyn Falco <[email protected]>

doc/how_tos/CreateCustomNamer.md

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+<a id="top"></a>
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+# How to Create a Custom Namer - NOT IMPLEMENTED YET!
+
+toc
+
+## Overview
+
+Warning - this is NOT IMPLEMENTED YET!
+
+The easiest way to create a custom namer is by modifying the parts of CustomizableNamer.
+
+CustomizableNamer has two main methods:
+
+* `getApprovedFile(std::string extensionWithDot)`
+This works by composing:
+`{TestFolderForApproved}`/`{RelativePathOfSourceDirectoryFromSourceRootForApproved}`/`{FileNameAndTestName}`.approved.`{FileExtension}`
+
+* `getReceivedFile(std::string extensionWithDot)`
+This works by composing:
+`{TestFolderForReceived}`/`{RelativePathOfSourceDirectoryFromSourceRootForReceived}`/`{FileNameAndTestName}`.received.`{FileExtension}`
+
+## Overriding Parts
+
+Both the getApprovedFile and getReceivedFile methods can be overridden, as well as any of the composable parts. By default, anything with a for received/apprroved simply calls the base.
+
+For example:
+
+```cpp
+auto myNamer = CustomizableNamer()
+    .withTestFolder([]{return "C:/ApprovalFiles";})
+    .withTestFolderForReceived([]{return getTestFolder() + "/received"});
+
+Approvals::verify("Hello World", Options().withNamer(myNamer));
+```
+
+{TestFolder}/{RelativePathOfSourceDirectoryFromSourceRoot}/{FileNameAndTestName}.approved.{FileExtension}
+
+{TestFolder}/{ReceivedOrApproved}/{RelativePathOfSourceDirectoryFromSourceRoot}/{SourceFileName}-{SectionNames}.{FileExtension}
+
+---
+
+[Back to User Guide](/doc/README.md#top)