Explain localization process (#6723)

Steps for localization process not clearly explained in the documentation

Author: kant2002

docs/building.md

@@ -62,6 +62,13 @@ Note that this does **not** build using your machine-wide installed version of t
 
 To create the Microsoft.Private.Winforms package, run `.\build -pack`
 
+## Localization
+
+If you need to add a new localization string or update an existing one, follow these steps:
+
+- Modify `Resource\SR.resx` file adding or updating necessary strings in the project that contains the said strings. (It is often faster/easier to open SR.resx in an XML editor).
+- Regenerate the localization files by rebuilding the solution/project from Visual Studio, or by executing `.\build.cmd` command. You can also build just the modified project by running  `dotnet build` from the project's root.
+
 [comment]: <> (URI Links)
 
 [corefx-windows-instructions]: https://github.com/dotnet/corefx/blob/master/Documentation/building/windows-instructions.md