Fix #145 provide more information in README re API and impl JARs

Signed-off-by: Mark Thomas <markt@apache.org>

README.md

@@ -1,24 +1,46 @@
 # Jakarta Expression Language
 
-This repository contains the code for Jakarta Expression Language.
+This repository contains the source for:
 
-[Online JavaDoc](https://javadoc.io/doc/jakarta.el/jakarta.el-api/)
+ - the Jakarta Expression Language [API](https://javadoc.io/doc/jakarta.el/jakarta.el-api/) (/api) - 
+ - the Jakarta Expression Langauge [specification](https://jakarta.ee/specifications/expression-language/4.0/jakarta-expression-language-spec-4.0.html) (/spec)
+ - the Glassfish implementation of the Jakarta Expression Language API (/impl)
 
-Building
---------
+Note: From Jakarta EE 9 onwards the JAR containing the Glassfish implementation
+of the Jakarta Expression Language API only contains the implementation classes.
+It no longer contains a copy of the API classes which may be obtained from the
+[API JAR](https://search.maven.org/artifact/jakarta.el/jakarta.el-api).
 
-Jakarta Expression Language can be built by executing the following from the project root:
+Note: The Glassfish implementation is expected to move to a new repository shortly.
 
-``mvn clean package``
 
-The API jar can then be found in /target.
+## Building
 
-Making Changes
---------------
+### API
+
+Jakarta Expression Language API can be built by executing the following from the project root:
+
+```
+cd api
+mvn clean package
+```
+The API jar can then be found in `/impl/target`.
+
+### Specification
+
+Jakarta Expression Language specification can be built by executing the following from the project root:
+
+```
+cd spec
+mvn clean package
+```
+The API jar can then be found in `/spec/target`.
+
+
+## Making Changes
 
 To make changes, fork this repository, make your changes, and submit a pull request.
 
-About Jakarta Expression Language
--------------
+## About Jakarta Expression Language
 
 Jakarta Expression Language defines an expression language for Java applications.