doc: Add documentation on how to handle multiple contributor names vi…

…a .mailmap file

Sources/SPIManifest/Documentation.docc/MailMapFile.md

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+# Handling Multiple Contributor Names with .mailmap
+
+### Introduction
+
+In some repositories, contributors may commit from different devices or with 
+different configurations, leading to multiple names appearing for the same 
+contributor. For example, a repository might show “John Doe” and “Johnathan 
+Doe” as two separate contributors, even though they are the same person. 
+
+This guide explains how to use a __.mailmap__ file to normalize commits and 
+ensure a single contributor name appears in your Swift Package.
+
+### Solution
+A __.mailmap file__ allows contributors to map multiple email addresses and 
+names to a single canonical name and email address. [Git](https://git-scm.com/docs/gitmailmap#:~:text=mailmap%20exists%20at%20the%20toplevel,real%20names%20and%20email%20addresses.) 
+provides a description of what a __.mailmap__ file is and a good example 
+can be found in [Swift-NIO](https://github.com/apple/swift-nio/blob/main/.mailmap).
+
+#### Step-by-Step Guide
+1. Create a .mailmap file in the root of your repository.
+2. Add mappings in the .mailmap file to normalize the contributor names and 
+email addresses. 
+   - The format is: `Proper Name <[email protected]> <[email protected]>`
+      - example: `John Doe <[email protected]> <[email protected]>`
+      - this is an example of a __.mailmap__ file:
+      ```
+         John Doe <[email protected]> <[email protected]>
+         John Doe <[email protected]> <[email protected]>
+      ```
+3. Add and commit the __.mailmap__ file:
+ ```
+ git add .mailmap
+ git commit -m "Add .mailmap to normalize contributor names"
+ git push origin main
+ ```
+
+4. Once the __.mailmap__ file is in place, the contributor list should reflect 
+the normalized names. 
+
+### Additional Configuration
+If the .mailmap file does not completely resolve the issue, you can override the
+ contributor list directly in the manifest. You can specify the contributors in 
+ the manifest as follows:
+ ```swift
+ .metadata.authors([
+    .author(name: "John Doe", email: "[email protected]")
+])
+```
+For more details refer to the [SPI Manifest Documentation](https://swiftpackageindex.com/swiftpackageindex/spimanifest/0.17.2/documentation/spimanifest/manifest/metadata-swift.struct/authors)