Add workflow/mapping.md to docs [skip ci]

- explain mapping concept
- describes mapping.json format
- overview function to work wiht mapping.json files from PowerShellGitHubDev.psm1

docs/workflow/mapping.md

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+# Mapping
+
+PowerShell/PowerShell utilizes `dotnet cli` project model.
+Source code for a library (executable) is located under `src/<library-name>`.
+I.e. System.Management.Automation.dll sources are located under `src/System.Management.Automation`
+
+In the windows source tree, this files located differently.
+That's why we have `mapping.json` in the root of PowerShell/PowerShell repo.
+This file is a simple json hashtable that describes mapping between files in source depot and GitHub.
+
+Keys are relative file paths from `src\monad` submodule (that has the same layout as admin sd enlistment).
+Values are relative file paths in PowerShell/PowerShell GitHub project.
+
+**Note**: this "src\monad" prefix appears in keys for historical reasons.
+We used to have a submodule at this path.
+If you replace this `src\monad` with path to the **admin** enlistment, you will get the mapping to source depot.
+
+### PowerShellGitHubDev.psm1
+
+Our dev module contains a number of functions to work that can be used to work with this mapping file.
+
+* `Copy-SubmoduleFiles` -- is used for "sd -> github" integration
+* `New-MappingFile` -- was used to create the first version on mapping.json
+* `Send-GitDiffToSd` -- the most interesting function for us: 
+it applies patch from git to **admin** enslistment with respect to `mapping.json`.
+It supports `-WhatIf` switch.
+
+```
+> Send-GitDiffToSd -diffArg1 45555786714d656bd31cbce67dbccb89c433b9cb -diffArg2 45555786714d656bd31cbce67dbccb89c433b9cb~1 -pathToAdmin d:\e\ps_dev\admin 
+> cd d:\e\ps_dev\admin
+> sd online ...
+> # move files to new change list (i.e. with sdb)
+> sd submit -c <n>
+ 
+```
+
+