Add a list of requirements/limitations to the documentation

[sendyne-nicocvn]

This is currently missing from the documentation and should include:

• supported compilers: GCC/Clang ... should work on other C++11 compilers but no guarantee,
• supported hardware: requires that register minimal size is 8 bit, designed for Cortex-M{0+,3,4,7} originally but nothing specifics so this should be generic enough to run on virtually any MCUs.

[sendyne-nicocvn]

So the main limitations are:

• registers sizes are an integral number of bytes,
• registers are properly aligned with respect to their sizes (i.e., a 16-bit register is aligned on a 16-bit boundary).

As far as compilers only GCC and Clang are guaranteed to work fine. Others should be fine most of the time if they are fully compliant with C++11/C++14.