Doc: Added set custom credentials instruction in README (#1461)

Author: owenowenisme

README.md

@@ -68,8 +68,11 @@ Apache Polaris is built using Gradle with Java 21+ and Docker 27+.
 - `./gradlew build` - To build and run tests. Make sure Docker is running, as the integration tests depend on it.
 - `./gradlew assemble` - To skip tests.
 - `./gradlew test` - To run unit tests and integration tests.
-- `./gradlew run` - To run the Polaris server locally; the server is reachable at 
-  localhost:8181. This is also suitable for running regression tests, or for connecting with Spark. 
+- `./gradlew run` - To run the Polaris server locally; the server is reachable at localhost:8181. This is also suitable for running regression tests, or for connecting with Spark. Set your own credentials by specifying system property `./gradlew run -Dpolaris.bootstrap.credentials=POLARIS,root,secret` where:
+  - `POLARIS` is the realm
+  - `root` is the CLIENT_ID
+  - `secret` is the CLIENT_SECRET
+  - If credentials are not set, it will use preset credentials `POLARIS,root,secret`
 - `./regtests/run_spark_sql.sh` - To connect from Spark SQL. Here are some example commands to run in the Spark SQL shell:
 ```sql
 create database db1;