update README file
* update README file
update VS 2010 as the minimal version of required Visual Studio
* Rephrase the MS VS requirement
Rephrase the VS version sentence
@@ -16,7 +16,7 @@ There are currently four active build systems used within mbed TLS releases:
- yotta
- GNU Make
- CMake
-- Microsoft Visual Studio (Visual Studio 6 and Visual Studio 2010)
+- Microsoft Visual Studio (Microsoft Visual Studio 2010 or later)
The main systems used for development are CMake and GNU Make. Those systems are always complete and up-to-date. The others should reflect all changes present in the CMake and Make build system, although features may not be ported there automatically.
