Update README.md

README.md

@@ -67,7 +67,7 @@ This file tells Augur the *sources* and *sinks* of the flows you want to
 track. The spec above tells Augur to alert you if any value returned from
 `readFileSync` flows into the function `exec`. It also tells Augur how to run
  your project: by executing the file `test.js`.
-[Here](./tests-unit/README.md) are all the options for `spec.json`.
+[Here](./tests-unit/README.md#specjson) are all the options for `spec.json`.
 
 Let's say we analyze the following program, `test.js`:
 ```javascript
@@ -135,6 +135,15 @@ You've now analyzed your first application using Augur!
 to full dependency information between variables
 5. Support for tracking taint through native code (see below)
 
+## Tracking Type
+Augur supports *three* methods for tracking taint across your application:
+
+1. `Boolean`: the simplest (and fastest) tracker you can use. During your application's runtime, it simply determines whether a value came from *any* source. It doesn't keep track of which source it came from, or where the flow was introduced. This is not very useful in practice, because you will likely want to use...
+2. `SourcedBoolean`: a more practical tracker. For each value in your program, Augur determines if it came from a *source*, and if so, which source and on what line the taint was introduced.
+3. `Expression`: the most general tracker. In this mode, Augur will save all the information it finds during your application's runtime. For any given expression, its full set of dependent expressions is recorded. In other words, regardless your specified sources and sinks, Augur will save *every* flow between *every* expression. Expect slowdowns and large output files (on the order of MBs).
+
+The method you choose should be placed in your [`spec.json`](./tests-unit/README.md#specjson).
+
 ## Native function models
 Modern JavaScript relies on a wide variety of native functions to improve
 its usability and performance. Common operations on data structures and