Add documentation about how to include the default omitted packages (#290)

Author: jdufresne

README.md

@@ -148,16 +148,35 @@ This will remove all folders within the installed requirements that match
 the names in `slimPatterns`  
 ## Omitting Packages 
 You can omit a package from deployment with the `noDeploy` option. Note that
-dependencies of omitted packages must explicitly be omitted too.
-By default, this will not install the AWS SDKs that are already installed on
-Lambda. This example makes it instead omit pytest:
+dependencies of omitted packages must explicitly be omitted too. By default,
+the following packages are omitted as they are already installed on Lambda:
+
+ * boto3
+ * botocore
+ * docutils
+ * jmespath
+ * pip
+ * python-dateutil
+ * s3transfer
+ * setuptools
+ * six
+
+This example makes it instead omit pytest:
 ```yaml
 custom:
   pythonRequirements:
     noDeploy:
       - pytest
 ```
 
+To include the default omitted packages, set the `noDeploy` option to an empty
+list:
+```yaml
+custom:
+  pythonRequirements:
+    noDeploy: []
+```
+
 ## Extra Config Options
 ### Caching
 You can enable two kinds of caching with this plugin which are currently both DISABLED by default.  First, a download cache that will cache downloads that pip needs to compile the packages.  And second, a what we call "static caching" which caches output of pip after compiling everything for your requirements file.  Since generally requirements.txt files rarely change, you will often see large amounts of speed improvements when enabling the static cache feature.  These caches will be shared between all your projects if no custom cacheLocation is specified (see below).