Added docs for floating point comparisons

docs/assertions.md

@@ -17,12 +17,12 @@ The ```CHECK``` family are equivalent but execution continues in the same test c
 Evaluates the expression and records the result. If an exception is thrown it is caught, reported, and counted as a failure. These are the macros you will use most of  the time
 
 Examples:
-
+```
-```c++
 CHECK( str == "string value" );
 CHECK( thisReturnsTrue() );
 REQUIRE( i == 42 );
 ```
+
 * **REQUIRE_FALSE(** _expression_ **)** and  
 * **CHECK_FALSE(** _expression_ **)**
 
@@ -30,10 +30,28 @@ Evaluates the expression and records the _logical NOT_ of the result. If an exce
 (these forms exist as a workaround for the fact that ! prefixed expressions cannot be decomposed).
 
 Example:
-```c++
+```
 REQUIRE_FALSE( thisReturnsFalse() );
 ```
 
+### Floating point comparisons
+
+When comparing floating point numbers - especially if at least one of them has been computed - great care must be taken to allow for rounding errors and inexact representations.
+
+Catch provides a way to perform tolerant comparisons of floating point values through use of a wrapper class called ```Approx```. ```Approx``` can be used on either side of a comparison expression. It overloads the comparisons operators to take a tolerance into account. Here's a simple example:
+
+```
+REQUIRE( performComputation() == Approx( 2.1 ) );
+```
+
+By default a small epsilon value is used that covers many simple cases of rounding errors. When this is insufficent the epsilon value (the amount within which a difference either way is ignored) can be specified by calling the ```epsilon()``` method on the ```Approx``` instance. e.g.:
+
+```
+REQUIRE( 22/7 == Approx( 3.141 ).epsilon( 0.01 ) );
+```
+
+When dealing with very large or very small numbers it can be useful to specify a scale, which can be achieved by calling the ```scale()``` method on the ```Approx``` instance.
+
 ## Exceptions
 
 * **REQUIRE_THROWS(** _expression_ **)** and