Work towards #56: Splitting up the README

I did the job and splitted up the readme, hopefully everything was
splitted correctly...

intercepting-filter/index.md

@@ -0,0 +1,18 @@
+---
+layout: pattern
+title: Intercepting Filter
+folder: intercepting-filter
+categories: pattern_cat
+tags: pattern_tag
+---
+
+**Intent:** Provide pluggable filters to conduct necessary pre-processing and
+post-processing to requests from a client to a target
+ 
+![alt text](./etc/intercepting-filter.png "Intercepting Filter")
+ 
+**Applicability:** Use the Intercepting Filter pattern when
+
+* a system uses pre-processing or post-processing requests
+* a system should do the authentication/ authorization/ logging or tracking of request and then pass the requests to corresponding handlers 
+* you want a modular approach to configuring pre-processing and post-processing schemes