Improve Building from Source Instructions

.changes/next-release/feature-AWSSDKforJavav2-edd67d8.json

@@ -0,0 +1,6 @@
+{
+    "type": "feature",
+    "category": "AWS SDK for Java v2",
+    "description": "Improve Building from Source Instructions",
+    "contributor": "helloworldless"
+}

README.md

@@ -116,16 +116,51 @@ See the [Set up the AWS SDK for Java][docs-setup] section of the developer guide
 
 Once you check out the code from GitHub, you can build it using Maven.
 
+Note: The `archetypes/archetype-lambda` project requires that you have Python 3 in your path as `python`.
+
+### Building a Specific Service Module
+
+This is the fastest option if you only need to build a certain sub-project like S3 or the DynamoDB Enhanced Client.
+
+```sh
+# Build a specific service module; Skip tests, checkstyles, findbugs, etc
+mvn clean install -pl :s3 -P quick --am
+```
+
+### Building the Entire Project
+
+Note that this can take quite a long time to complete.
+
 ```sh
+# Build the entire project
 mvn clean install
 
-# Skip tests, checkstyles, findbugs, etc for quick build
+# Build the entire project; Skip tests, checkstyles, findbugs, etc
 mvn clean install -P quick
-
-# Build a specific service module
-mvn clean install -pl :s3 -P quick --am
 ```
 
+### Code Generation
+
+The SDK makes significant use of code generation which takes place at build time. During a Maven build, the 
+`build-helper-maven-plugin` takes care of making these additional generated source files available for compilation. 
+
+However, your IDE may not automatically detect the generated code which can lead to class not found errors. 
+To fix this, you may need to instruct your IDE where to find the generated code: namely, in the 
+`target/generated-sources` of many of the projects in this repo.
+
+In IntelliJ IDEA, the default setting of "Generated sources folders: Detect automatically"
+under `File | Settings | Build, Execution, Deployment | Build Tools | Maven | Importing` should suffice. 
+However, you may still have to force a generated sources update by opening  
+`View | Tool Windows | Maven` and clicking the second icon from the left: 
+"Generate Sources and Update Folders for All Projects". Alternatively, if there is a specific project you would like  
+to build and you know which projects' generated code it depends on, you can force a generated sources update just for 
+those projects which will be much faster that generating sources for the entire project. For example, 
+the DynamoDB Enhanced Client (`services-custom/dynamodb-enhanced`) depends 
+directly or indirectly on code generated by DynamoDB (`services/dynamodb`), SDK Core (`core/sdk-core`), 
+and Regions (`core/regions`). So in this case, you can just find 
+"AWS Java SDK :: Services :: Amazon DynamoDB", "AWS Java SDK :: SDK Core", and "AWS Java SDK :: Regions" 
+in the Maven toolbar, right click on each, and choose "Generate Sources and Update Folders".
+
 ## Sample Code
 You can find sample code for v2 in the following places: