Update README.md for Service Principal Installation Limit (#1076)

Added the detail that running UCX as a Service Principal is not
supported under Installation

## Changes
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### Linked issues
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Resolves #..

### Functionality 

- [ ] added relevant user documentation
- [ ] added new CLI command
- [ ] modified existing command: `databricks labs ucx ...`
- [ ] added a new workflow
- [ ] modified existing workflow: `...`
- [ ] added a new table
- [ ] modified existing table: `...`

### Tests
<!-- How is this tested? Please see the checklist below and also
describe any other relevant tests -->

- [ ] manually tested
- [ ] added unit tests
- [ ] added integration tests
- [ ] verified on staging environment (screenshot attached)

Author: NateAnthBricks

README.md

@@ -63,7 +63,7 @@ See [contributing instructions](CONTRIBUTING.md) to help improve this project.
 - Python 3.10 or later. See [Windows](https://www.python.org/downloads/windows/) instructions.
 - Network access to your Databricks Workspace used for the [installation process](#install-ucx).
 - Network access to the Internet for [pypi.org](https://pypi.org) and [github.com](https://github.com) from machine running the installation.
-- Databricks Workspace Administrator privileges for the user, that runs the installation.
+- Databricks Workspace Administrator privileges for the user, that runs the installation. Running UCX as a Service Principal is not supported.
 - Account level Identity Setup. See instructions for [AWS](https://docs.databricks.com/en/administration-guide/users-groups/best-practices.html), [Azure](https://learn.microsoft.com/en-us/azure/databricks/administration-guide/users-groups/best-practices), and [GCP](https://docs.gcp.databricks.com/administration-guide/users-groups/best-practices.html).
 - Unity Catalog Metastore Created (per region). See instructions for [AWS](https://docs.databricks.com/en/data-governance/unity-catalog/create-metastore.html), [Azure](https://learn.microsoft.com/en-us/azure/databricks/data-governance/unity-catalog/create-metastore), and [GCP](https://docs.gcp.databricks.com/data-governance/unity-catalog/create-metastore.html).
 - If your Databricks Workspace relies on an external Hive Metastore (such as AWS Glue), make sure to read the [this guide](docs/external_hms_glue.md).