group pattern-* related papers (#564)

* combine clustering algo into pattern matching

* rename stringology with the pattern_ prefix

* improved the README header info for paper related to patterns

clustering_algorithms/README.md

@@ -1,3 +0,0 @@
-## Clustering Algorithms
-
-[On the resemblance and containment of documents](https://www.cs.princeton.edu/courses/archive/spr05/cos598E/bib/broder97resemblance.pdf) (Andrei Z. Broder)

pattern_matching/README.md

@@ -1,6 +1,19 @@
 # Pattern Matching
 
+[Pattern matching algorithms](https://en.wikipedia.org/wiki/Pattern_matching) extend beyond strings, to include trees and other data structures. 
+
+🧭 Part of the pattern-matching group, in PWL
+
+* [Pattern Matching](../pattern_matching/README.md)
+* [Stringology](../pattern_stringology/README.md)
+
+----
+
 * :scroll: [Compiling Pattern Matching to good Decision Trees](compiling-pattern-matching-to-good-decision-trees.pdf) by Luc Maranget. Paper address the issue of compiling ML pattern matching to efficient decisions trees.
 * :scroll: [Extensible Pattern Matching in an Extensible Language](extensible-pattern-matching-extensible-language.pdf) by Sam Tobin-Hochstadt. Paper present a sophisticated pattern matcher for [Racket](http://racket-lang.org/), implemented as language extension using macros.
 * :scroll: [Warnings for pattern matching](warnings-for-pattern-matching.pdf) by Luc Maranget. Paper examine the ML pattern-matching anomalies of useless clauses and non-exhaustive matches.
 * :scroll: [Efficient String Matching: An Aid to Bibliographic Search](aho-corasick-string-matching.pdf) by Alfred V. Aho and Margaret J. Corasick. A single-pass pattern matching algorithm used in early versions of fgrep and similar projects. [Aho–Corasick entry](http://xlinux.nist.gov/dads//HTML/ahoCorasick.html) in NIST's Dictionary of Algorithms and Data Structures.
+
+## Clustering Algorithms
+
+* [On the resemblance and containment of documents](https://www.cs.princeton.edu/courses/archive/spr05/cos598E/bib/broder97resemblance.pdf) (Andrei Z. Broder)

pattern_stringology/README.md

@@ -1,5 +1,13 @@
 # Stringology
 
+[Stringology](https://en.wikipedia.org/wiki/String_(computer_science)#String_processing_algorithms) refers to one of the various kinds of string processing algorithms. 
+
+
+🧭 Part of the pattern-matching group, in PWL
+
+* [Pattern Matching](../pattern_matching/README.md)
+* [Stringology](../pattern_stringology/README.md)
+
 ## External Papers
 
 * [A Taxonomy of Suffix Array Construction Algorithms](http://www.cas.mcmaster.ca/~bill/best/algorithms/07Taxonomy.pdf)