docs: update readme with mapping capability (#114)

### Description

This PR adds the new mapping capability to the README.md file. 

**Issue number:** #80 

### Type of change

- [x] New feature (non-breaking change which adds functionality)
- [ ] Fix (non-breaking change which fixes an issue)
- [ ] Breaking change (fix or feature that would cause existing
functionality to not work as expected)

### Checklist

- [x] I have performed a self-review of this change
- [x] Changes have been tested
- [x] Changes are documented
- [x] Changes generate *no new warnings*
- [x] PR title follows [conventional commit
semantics](https://www.conventionalcommits.org/en/v1.0.0/)

If this is a breaking change 👇

- [ ] I have documented the migration process
- [ ] I have implemented necessary warnings (if it can live side by
side)

## Acknowledgement

By submitting this pull request, I confirm that you can use, modify,
copy, and redistribute this contribution, under the terms of your
choice.

**Disclaimer:** We value your time and bandwidth. As such, any pull
requests created on non-triaged issues might not be successful.

README.md

@@ -13,6 +13,7 @@ The PagerDuty plugin reduces the cognitive load on developers responsible for ma
 - **[See Active Incidents](<https://pagerduty.github.io/backstage-plugin-docs/capabilities/#view-any-open-incidents>)** - View all active incidents for a service directly in Backstage.
 - **[Check for Recent Changes](<https://pagerduty.github.io/backstage-plugin-docs/capabilities/#view-change-events-associated-to-a-service>)** - Identify recent changes that may be the root cause of potential issues with your service.
 - **[Identify On-Call Personnel](<https://pagerduty.github.io/backstage-plugin-docs/capabilities/#see-and-contact-on-call-staff>)** - Quickly determine who is responsible for a failing service and resolve the incident as quickly as possible. This feature allows companies to spend more time solving problems rather than determining who should solve them.
+- **Map existing PagerDuty services to Backstage entities** - Leverage PagerDuty's Advanced Configuration page to map existing PagerDuty services to Backstage entities without code changes.
 
 ## Getting Started