doc: update README.md to notice the user about RisingWave's usage of Scarf

[alicelyy]

A few examples of other open source projects using similar communication:

• https://github.com/apache/superset/wiki/Superset-Third%E2%80%90Party-Dependencies
• https://github.com/ngx-lottie/ngx-lottie
• https://github.com/apache/sedona/blob/c878485ef7f5941baa2871d6f0cc03b782e41a5c/docs/asf/telemetry.md?plain=1
• https://github.com/search?q=%22https%3A%2F%2Fscarf.sh%2F%22+language%3AMarkdown&type=code&l=Markdown

Our telemetry communication:
https://docs.risingwave.com/docs/current/telemetry/#how-to-opt-out

[xiangjinwu]

There is a Notes on telemetry section below already.

RisingWave collects anonymous usage statistics to better understand how the community is using RisingWave. The sole intention of this exercise is to help improve the product. Users may opt out easily at any time. Please refer to the user documentation for more details.

[alicelyy]

There is a Notes on telemetry section below already.

RisingWave collects anonymous usage statistics to better understand how the community is using RisingWave. The sole intention of this exercise is to help improve the product. Users may opt out easily at any time. Please refer to the user documentation for more details.

Telemetry is not our only usage of Scarf. We also track user IP from docker download link (pending CNAME change from our network infra provider) as well as various website pages visits. For this reason I think it may be more appropriate to put the notice at a higher level location.