Update readme

README.md

@@ -13,10 +13,47 @@ These operations are heavily used in low-level implementations of databases
 for their indexing logic, but has applications with statistical analysis and
 other domains that require logical set operations.
 
-This seems very specific to only use u64, but has been chosen for a good reason. On
-64bit cpus, native 64bit operations are faster than 32/16. Additionally,
-due to the design of the library, unsigned types are simpler to operate
-on for the set operations.
+How Does It Work?
+-----------------
+
+Each set initially is "sparse". This is stored in the manner you expect historically,
+using a `Vec<u64>` internally.
+
+::
+
+    [ 0, 1, 2, 3, ... , 1024 ]
+
+You can then call `maybe_compress` on the set, which will look at the content and determine
+if it would be beneficial to compress this. When compressed, each value is transformed into
+a tuple pair of `range` and `mask`. The range represents the starting value of this set of
+64 values, and the mask determines if a value of that range is present. For example:
+
+<p align="center">
+	<img src="https://raw.githubusercontent.com/Firstyear/idlset/master/static/idl_4.png" width="60%" height="auto" />
+</p>
+
+As these now contain a bit mask, we can use CPU operations for logical operations like `AND`, `OR` and
+`AND NOT`. This example demonstrates an `AND` operation.
+
+<p align="center">
+	<img src="https://raw.githubusercontent.com/Firstyear/idlset/master/static/idl_5.png" width="60%" height="auto" />
+</p>
+
+Due to this compression, on high density sets, memory is reduced, as is improvements to CPU cache
+behaviour due to lower pressure on the caches. It also allows faster seeking through sets to determine
+value presence.
+
+<p align="center">
+	<img src="https://raw.githubusercontent.com/Firstyear/idlset/master/static/idl_6.png" width="60%" height="auto" />
+</p>
+
+During operations between compressed and uncompressed sets, the "better" choice of compressed or
+uncompressed is preserved for the result set based on the inputs and operation performed.
+In other words, the result set may be compressed or uncompressed
+depending on the operation and it's interactions, to improve performance of subsequent operations.
+This helps to carry forward these optimisation choices to result sets meaning that chained and
+many operations over sets, and reduces memory consumption of intermediate set results during
+operations.
 
 Contributing
 ------------