adding info about clowder/generator repo to README.md (#35)

* adding info about clowder/generator repo and mentioning onbuild is no longer maintained

Author: Michael-D-Johnson

README.md

@@ -216,8 +216,13 @@ to a file that is read with the configuration options.
 
 # Dockerfile
 
-We recommend using the pyclowder:onbuild to easily convert your extractor into a docker container. If you build the
-extractor as commented above, you will only need the following Dockerfile
+We recommend following the instructions at [clowder/generator](https://github.com/clowder-framework/generator) to build a Docker image from your Simple Extractor.
+
+You can also use the pyclowder:onbuild Docker image to easily convert your extractor into a docker container. This image is no longer maintained so it is recommeded to either use the clowder/generator linked above or build your own Dockerfile by choosing your own base image and installing pyClowder as described below.
+
+
+**This is deprecated and the onbuild image is no longer maintained**
+If you build the extractor as using the pyclowder:onbuild image, you will only need the following Dockerfile
 
 ```
 FROM clowder/pyclowder:onbuild
@@ -290,7 +295,7 @@ def wordcount(input_file):
     return result
 ```
 
-To build wordcount as a Simpel extractor docker image, users just simply assign two environment variables in Dockerfile shown below. EXTRACTION_FUNC is environment variable and has to be assigned as extraction function, where in wordcount.py, the extraction function is `wordcount`. Environment variable EXTRACTION_MODULE is the name of module file containing the definition of extraction function.
+To build wordcount as a an extractor docker image, users just simply assign two environment variables in Dockerfile shown below. EXTRACTION_FUNC is environment variable and has to be assigned as extraction function, where in wordcount.py, the extraction function is `wordcount`. Environment variable EXTRACTION_MODULE is the name of module file containing the definition of extraction function.
 ```markdown
 FROM clowder/extractors-simple-extractor:onbuild