Update teensy README.md file

Thanks to Artur Wroblewski for some suggested changes.
I also added the TIPs section at the end while I was updating.
Merge pull request #841 from dhylands/teensy-README

Update teensy README.md file
Added 'drivers' directory, intended to hold code for specific hardware.
drivers: Initial import of WIZnet5x000 driver.
drivers, wiznet5k: Fix IP addr verification.
drivers, wiznet5k: Change SPI interface to read/write multiple bytes.
drivers, wiznet5k: Add W5200 support.
drivers, wiznet5k: Properly fix ARP bug with W5200 chipset.
drivers, wiznet5k: Add HAL_Delay(1) to "infinite" loops.
drivers, wiznet5k: Make DNS service use HAL sys tick.
stmhal: Add wiznet5k module, to control WIZnet ethernet adaptor.

Allows to create socket objects that support TCP and UDP in server and
client mode.  Interface is very close to standard Python socket class,
except bind and accept do not work the same (due to hardware not
supporting them in the usual way).

Not compiled by default.  To compile this module, use:
make MICROPY_PY_WIZNET5K=1
stmhal, modwiznet5k: Add very minimal documentation.
msvc: Exclude modtermios, include extmod and fix compilation error
Merge branch 'msvc-extmod' of github.com:stinos/micropython into stinos-msvc-extmod
extmod: Fix type-punned-ptr error.
Merge branch 'stinos-msvc-extmod'
Add cache flush in py/asmarm.c and add new MP_PLAT_ALLOC_EXEC and MP_PLAT_FREE_EXEC macros

Fixes issue #840