New documentation section: "known limitations"

It contains known limitation such as the fact that Catch is not thread
safe, it does not support running tests in forked process or running
multiple tests in parallel

closes #399
closes #681
closes #246
closes #483

docs/Readme.md

@@ -12,6 +12,7 @@ Before looking at this material be sure to read the [tutorial](tutorial.md)
 * [Configuration](configuration.md)
 * [String Conversions](tostring.md)
 * [Why are my tests slow to compile?](slow-compiles.md)
+* [Known limitations](limitations.md)
 
 Other