Added some documentation on how Adaptive Forms submission handling works

Author: rmcdouga

spring/AdaptiveFormSubmissionHandling.md

@@ -0,0 +1,23 @@
+# Adaptive Form Submission Handling in fluent-forms-spring-boot-starter
+
+The submission handling code looks for one or more beans in the Spring Context that provide the 
+`AfSubmissionHandler` interface.  This interface has two methods `boolean canHandle(String formName)`
+ and `SubmitResponse processSubmission(Submission submission)`.  Whenever a submission comes in, the 
+ `canHandler` method is called on each `AfSubmissionHandler` bean – passing in the Adaptive Form’s name. 
+The first `AfSubmissionHandler` that returns `true` (to indicate it can handle this form’s submission) 
+will have its `processSubmission()` method called.
+
+The `processSubmission()` method receives a `Submission` object (which is populated with the submission’s 
+information such as the form data, form name, http headers and the submission redirect URL).  The 
+`processSubmission()` method returns a `SubmitResponse` interface which contains information that will be 
+placed in the response.  It can currently be a `Response` object, a `SeeOther` object or a `Redirect` 
+object (which all implement the `SubmitResponse` interface).  A `Response` object is for returning text, 
+html, json, etc. to the browser.  A `SeeOther` will redirect the browser to perform a GET on another URL.  A `Redirect` will redirect the browser to perform a POST to another URL.
+
+There are a bunch of static convenience methods in `AfSubmissionHandler` that means that you probably 
+won’t have to create an `AfSubmissionhandler` directly, but can use a convenience method like 
+`AfSubmissionHandler. canHandleFormNameEquals(String formName, Function<Submission, SubmitResponse> handlerLogic)` 
+and others.
+
+There are also static convenience functions in the `SubmitResponse.Response` object that make it easy to 
+create the most common responses (e.g. `Response json(String json)`).