Adds a note about modern add-ins

[mscharlock]

I work on the docs for Office Add-ins (https://github.com/OfficeDev/office-js-docs-pr) and by studying web analytics data, we think that adding a note to select high-trafficked pages of this docset will drive customers to a more modern add-ins solution, if they want that. Data from our customer surveys and interviews show that most people who have used previous versions of related Office technology do not discover the modern Office Add-in docs, so we are trying to drive discoverability. Happy to answer more questions on this if you have them.

[o365devx]

Docs Build status updates of commit 54d1033:

❌ Validation status: errors

Please follow instructions here which may help to resolve issue.

File	Status	Preview URL	Details
docs/about-the-open-xml-sdk.md	❌Error		Details
docs/getting-started.md	❌Error		Details
docs/how-to-parse-and-read-a-large-spreadsheet.md	❌Error		Details
docs/open-xml-sdk.md	❌Error		Details
docs/structure-of-a-spreadsheetml-document.md	❌Error		Details

docs/about-the-open-xml-sdk.md

• [Error] Cannot resolve '../includes/addinsnote.md' relative to 'about-the-open-xml-sdk.md'.

docs/getting-started.md

• [Error] Cannot resolve '../includes/addinsnote.md' relative to 'getting-started.md'.

docs/how-to-parse-and-read-a-large-spreadsheet.md

• [Error] Cannot resolve '../includes/addinsnote.md' relative to 'how-to-parse-and-read-a-large-spreadsheet.md'.

docs/open-xml-sdk.md

• [Error] Cannot resolve '../includes/addinsnote.md' relative to 'open-xml-sdk.md'.

docs/structure-of-a-spreadsheetml-document.md

• [Error] Cannot resolve '../includes/addinsnote.md' relative to 'structure-of-a-spreadsheetml-document.md'.

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.

[o365devx]

Docs Build status updates of commit cc932ee:

❌ Validation status: errors

Please follow instructions here which may help to resolve issue.

File	Status	Preview URL	Details
docs/about-the-open-xml-sdk.md	❌Error		Details
docs/getting-started.md	❌Error		Details
docs/how-do-i.md	❌Error		Details
docs/how-to-parse-and-read-a-large-spreadsheet.md	❌Error		Details
docs/open-xml-sdk.md	❌Error		Details
docs/structure-of-a-spreadsheetml-document.md	❌Error		Details

docs/about-the-open-xml-sdk.md

• [Error] Cannot resolve '../includes/addinsnote.md' relative to 'about-the-open-xml-sdk.md'.

docs/getting-started.md

• [Error] Cannot resolve '../includes/addinsnote.md' relative to 'getting-started.md'.

docs/how-do-i.md

• [Error] Cannot resolve '../includes/addinsnote.md' relative to 'how-do-i.md'.

docs/how-to-parse-and-read-a-large-spreadsheet.md

• [Error] Cannot resolve '../includes/addinsnote.md' relative to 'how-to-parse-and-read-a-large-spreadsheet.md'.

docs/open-xml-sdk.md

• [Error] Cannot resolve '../includes/addinsnote.md' relative to 'open-xml-sdk.md'.

docs/structure-of-a-spreadsheetml-document.md

• [Error] Cannot resolve '../includes/addinsnote.md' relative to 'structure-of-a-spreadsheetml-document.md'.

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.

[o365devx]

Docs Build status updates of commit 05cd9ee:

✅ Validation status: passed

File	Status	Preview URL	Details
docs/about-the-open-xml-sdk.md	✅Succeeded
docs/getting-started.md	✅Succeeded
docs/how-do-i.md	✅Succeeded
docs/how-to-parse-and-read-a-large-spreadsheet.md	✅Succeeded
docs/includes/addinsnote.md	✅Succeeded
docs/open-xml-sdk.md	✅Succeeded
docs/structure-of-a-spreadsheetml-document.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.