Update instructions in the the README for building reference docs

The instructions should have been updated as part of the work on
gh-5267 so that the zip of all of the starter poms is available to
the documentation's build. With that fixed, the current instructions
still fail as the javadoc fails to generated as a result of
org.springframework.boot:spring-boot:test-jar:tests being unavailable.
This can be avoid by simply not trying to build the javadoc, i.e.
by running generate-resources rather than install.

Closes gh-5633

README.adoc

@@ -127,11 +127,19 @@ available so you need to build that first since it's not generated by default.
 	$ ./mvnw clean install -pl spring-boot-tools/spring-boot-maven-plugin -Pdefault,full
 ----
 
+The documentation also includes auto-generated information about the starter poms. To
+allow this information to be collected, the starter projects must be built first:
+
+[indent=0]
+----
+	$ ./mvnw clean install -f spring-boot-starters
+----
+
 Once this is done, you can build the reference documentation with the command below:
 
 [indent=0]
 ----
-	$ ./mvnw clean install -pl spring-boot-docs -Pdefault,full
+	$ ./mvnw clean generate-resources -pl spring-boot-docs -Pdefault,full
 ----
 
 TIP: The generated documentation is available from `spring-boot-docs/target/contents/reference`