Added multi-level scanning/merging use case

[grokky1]

Explanation of how to make msbuild scan and merge multiple levels of Directory.Build.props files, for more complex solution structures. Related to 2456.

[msftclas]

@grokky1,
Thanks for your contribution.
To ensure that the project team has proper rights to use your work, please complete the Contribution License Agreement at https://cla.microsoft.com.

It will cover your contributions to all Microsoft-managed open source projects.
Thanks,
Microsoft Pull Request Bot

[saldana]

✅ Validation status: passed

File	Status	Preview URL	Details
docs/msbuild/customize-your-build.md	✅Succeeded

For more details, please refer to the build report.

Note: If you changed an existing file name or deleted a file, broken links in other files to the deleted or renamed file are listed only in the full build report.

[saldana]

✅ Validation status: passed

File	Status	Preview URL	Details
docs/msbuild/customize-your-build.md	✅Succeeded

For more details, please refer to the build report.

Note: If you changed an existing file name or deleted a file, broken links in other files to the deleted or renamed file are listed only in the full build report.

[Mikejo5000]

Made some minor edits

[saldana]

✅ Validation status: passed

File	Status	Preview URL	Details
docs/msbuild/customize-your-build.md	✅Succeeded

For more details, please refer to the build report.

Note: If you changed an existing file name or deleted a file, broken links in other files to the deleted or renamed file are listed only in the full build report.

[grokky1]

@Mikejo5000 There's a problem with the syntax, see for more info.

I made a new PR to address that issue: #278