[developer docs] Improve instructions to setup Pulsar development in …

…IntelliJ (apache#8892)

### Motivation

The current instructions to setup Pulsar development in IntelliJ could be improved. 

### Modifications

Update README.md
* improved instructions for Pulsar development in IntelliJ
* update the information that building Pulsar with newer than Java 8 has issues (see the changes for details)

README.md

@@ -86,8 +86,9 @@ components in the Pulsar ecosystem, including connectors, adapters, and other la
 ## Build Pulsar
 
 Requirements:
- * Java JDK 1.8 or Java JDK 11
- * Maven 3.3.9+
+ * Java 8 JDK (for building Pulsar)
+   * When building Pulsar on a higher version (higher than Java 8), the resulting artifacts are not compatible with Java 8 runtime because of some issues, such as [issue 8445](https://github.com/apache/pulsar/issues/8445).
+ * Maven 3.6.1+
 
 Compile and install:
 
@@ -135,7 +136,7 @@ required plugins.
 
 ### Intellij
 
-To configure annotation processing in IntelliJ:
+#### Configure annotation processing in IntelliJ
 
 1. Open Annotation Processors Settings dialog box by going to
    `Settings -> Build, Execution, Deployment -> Compiler -> Annotation Processors`.
@@ -153,6 +154,26 @@ To configure annotation processing in IntelliJ:
 
 5. Install the lombok plugin in intellij.
 
+#### Further configuration in IntelliJ 
+
+* When working on the Pulsar core modules in IntelliJ, reduce the number of active projects in IntelliJ to speed up IDE actions and reduce unrelated IDE warnings.
+  * In IntelliJ's Maven UI's tree view under "Profiles"
+    * Activate "core-modules" Maven profile
+    * De-activate "main" Maven profile
+    * Run the "Reload All Maven Projects" action from the Maven UI toolbar. You can also find the action by the name in the IntelliJ "Search Everywhere" window that gets activated by pressing the **Shift** key twice.
+
+* Run the "Generate Sources and Update Folders For All Projects" action from the Maven UI toolbar. You can also find the action by the name in the IntelliJ "Search Everywhere" window that gets activated by pressing the **Shift** key twice. Running the action takes about 10 minutes for all projects. This is faster when the "core-modules" profile is the only active profile.
+
+
+#### IntelliJ usage tips
+
+* In the case of compilation errors with missing Protobuf classes, ensure to run the "Generate Sources and Update Folders For All Projects" action.
+
+* All of the Pulsar source code doesn't compile properly in IntelliJ and there are compilation errors.
+  * Use the "core-modules" profile if working on the Pulsar core modules since the source code for those modules can be compiled in IntelliJ.
+  * Sometimes it might help to mark a specific project ignored in IntelliJ Maven UI by right-clicking the project name and select **Ignore Projects** from the menu.
+  * Currently, it is not always possible to run unit tests directly from the IDE because of the compilation issues. As a workaround, individual test classes can be run by using the `mvn test -Dtest=TestClassName` command.
+
 ### Eclipse
 
 Follow the instructions [here](https://howtodoinjava.com/automation/lombok-eclipse-installation-examples/)
@@ -188,4 +209,3 @@ This distribution includes cryptographic software. The country in which you curr
 The U.S. Government Department of Commerce, Bureau of Industry and Security (BIS), has classified this software as Export Commodity Control Number (ECCN) 5D002.C.1, which includes information security software using or performing cryptographic functions with asymmetric algorithms. The form and manner of this Apache Software Foundation distribution makes it eligible for export under the License Exception ENC Technology Software Unrestricted (TSU) exception (see the BIS Export Administration Regulations, Section 740.13) for both object code and source code.
 
 The following provides more details on the included cryptographic software: Pulsar uses the SSL library from Bouncy Castle written by http://www.bouncycastle.org.
-