Explain memory settings

[phdmakk]

With the increasing size of some of the input TTCN-3 test suites, performance may suffer due to the rather conservative default upper memory limits. There may even be out of memory errors occurring, depending on the input size and the selected configuration. As a first step, the situation should be properly documented so that there is at least a temporary solution until a more appropriate one is found (in a separate ticket)