added explanation on the algorithm

README.md

@@ -1,4 +1,63 @@
-aho-corasick
+Aho-Corasick
 ============
 
-Java implementation of the Aho-Corasick algorithm for efficient string matching
+Introduction
+------------
+Nowadays most free-text searching is based on Lucene-like approaches, where the search text is parsed into its
+various components. For every keyword a lookup is done to see where it occurs. When looking for a couple of keywords
+this approach is great. But what about it if you are not looking for just a couple of keywords, but a 100,000 of
+them? Like, for example, checking against a dictionary?
+
+This is where the Aho-Corasick algorithm shines. Instead of chopping up the search text, it uses all the keywords
+to build up a construct called a [Trie](http://en.wikipedia.org/wiki/Trie). There are three crucial components
+to Aho-Corasick:
+* goto
+* fail
+* output
+
+Every character encountered is presented to a state object within the goto structure. If there is a matching state,
+that will be elevated to the new current state.
+
+However, if there is no matching state, the algorithm will fall back to states with less depth (ie, a match less long)
+and proceed from there, until it found a matching state, or it has reached the root state.
+
+Whenever a state is reached that matches an entire keyword, it is emitted to an output set which can be read after the
+entire scan has completed.
+
+The beauty of the algorithm is that it is O(n). No matter how many keywords you have, or how big the search text is,
+the performance will decline in a linear way.
+
+Some examples you could use the Aho-Corasick algorithm for:
+* looking for certain words in texts in order to URL link or emphasize them
+* adding semantics to plain text
+* checking against a dictionary to see if syntactic errors were made
+
+This library is the Java implementation of the afore-mentioned Aho-Corasick algorithm for efficient string matching.
+The algorithm is explained in great detail in the white paper written by Aho and Corasick:
+ftp://163.13.200.222/assistant/bearhero/prog/%A8%E4%A5%A6/ac_bm.pdf.
+
+Usage
+-----
+
+```java
+    Trie trie = new Trie();
+    trie.addKeyword("hers");
+    trie.addKeyword("his");
+    trie.addKeyword("she");
+    trie.addKeyword("he");
+    Collection<Emit> emits = trie.parseText("ushers");
+```
+
+License
+-------
+   Licensed under the Apache License, Version 2.0 (the "License");
+   you may not use this file except in compliance with the License.
+   You may obtain a copy of the License at
+
+	http://www.apache.org/licenses/LICENSE-2.0
+
+   Unless required by applicable law or agreed to in writing, software
+   distributed under the License is distributed on an "AS IS" BASIS,
+   WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+   See the License for the specific language governing permissions and
+   limitations under the License.