creates printabletypes.md to describe output

My findings after a morning of digging through the source.

docs/printabletypes.md

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+# Making Types Printable
+
+Catch is capable of using user defined print functions and overloads.
+
+## Using ostreams
+
+By default, catch will attempt to display types using an ostream overload is available.
+
+```c++
+std::ostream& operator<<(std::ostream&, MySpecialType const&);
+```
+
+## using toString
+If you don't want your type to work with ostreams, or Catch is unable to find the 
+overload you provide, you may alternatively overload `Catch::toString`.  The overload 
+must appear **above** the include of `catch.hpp`.  For example, if I want to provide
+a print functionality for `std::pair<int, char>` in my test, the top of my test file
+would look like this:
+
+```c++
+#include <string>
+#include <sstream>
+#include <utility>
+
+namespace Catch {
+std::string toString(std::pair<int, char> const p) {
+    std::ostringstream oss;
+    oss << '{' << p.first << ", " << p.second << '}';
+    return oss.str();
+}
+}
+
+#include "catch.hpp"
+```
+Note that the former approach is preferred, and the latter is more error-prone
+due to possible typos when typing `Catch` or `toString`
+
+## Default output
+
+If neither of the two conditions has been met, catch will output `{?}` by default.