Updated RC-related verbiage (#163)

Updated RC-related verbiage as per DOCS-6850.

<!-- CURSOR_SUMMARY -->
---

> [!NOTE]
> **Low Risk**
> Documentation-only changes; low risk beyond potential user confusion
if the updated run instructions are incorrect.
> 
> **Overview**
> Updates `README.md` run instructions for Linux/Intel Macs to
**separately document running the latest production image** vs running
the **SingleStore 9.1 release candidate** via
`SINGLESTORE_VERSION="rc:9.1"`.
> 
> Adds an explicit note for 9.1 RC users to remove old RC
images/containers and re-pull to ensure 9.1-specific features are
enabled.
> 
> <sup>Written by [Cursor
Bugbot](https://cursor.com/dashboard?tab=bugbot) for commit
3598626. This will update automatically
on new commits. Configure
[here](https://cursor.com/dashboard?tab=bugbot).</sup>
<!-- /CURSOR_SUMMARY -->

Author: cgl-memsql

README.md

@@ -48,14 +48,35 @@ When running a Docker container, either with or without a free edition license,
 
 *See next section for Apple silicon instructions.*
 
+If you want to run the latest production release of SingleStore:
+
+```bash
+docker run \
+    -d --name singlestoredb-dev \
+    -e ROOT_PASSWORD="YOUR SINGLESTORE ROOT PASSWORD" \
+    -p 3306:3306 -p 8080:8080 -p 9000:9000 \
+    ghcr.io/singlestore-labs/singlestoredb-dev:latest
+```
+
+If you want to run the latest SingleStore 9.1 release candidate (RC):
+
 ```bash
 docker run \
     -d --name singlestoredb-dev \
     -e ROOT_PASSWORD="YOUR SINGLESTORE ROOT PASSWORD" \
+    -e SINGLESTORE_VERSION="rc:9.1" \
     -p 3306:3306 -p 8080:8080 -p 9000:9000 \
     ghcr.io/singlestore-labs/singlestoredb-dev:latest
 ```
 
+> **Note**
+> 
+> If you had previously run a SingleStore 9.1 RC Dev image and found that 9.1-specific features were not working as expected (such as `fv_vector_f16_type`):
+> * Remove (delete) your existing 9.1 RC Dev image and container
+> * Run the latest 9.1 RC Dev image
+>
+> Performing these steps will ensure that all SingleStore 9.1 features are enabled in the 9.1 RC Dev image.
+
 If you want to configure a specific license, simply pass it as an environment variable:
 
 ```bash