Add better build documentation

[gmaxwell]

The library is designed to be easy to use without the build system, e.g. you can build the tests with nothing more than:

$ gcc -o tests src/tests.c -I. -Iinclude/ -DUSE_NUM_NONE -DUSE_FIELD_INV_BUILTIN -DUSE_SCALAR_INV_BUILTIN -DUSE_FIELD_10X26 -DUSE_SCALAR_8X32 -DVERIFY

We should document this fact better, because for many embedded systems the whole build system is overkill. See also #184

Related, we should also provide packaging instructions for common OSes, see #545

[real-or-random]

I'm not sure if we want this but this could be made even easier with just -DUSE_BASIC_CONFIG if we include basic-config.h at the appropriate places. It's only used for gen-context at the moment.

[gmaxwell]

We could also do some ifdef decision to use 32 vs 64 bit fields by default.

[real-or-random]

We currently rely on ./configure to define WORDS_BIGENDIAN, which could lead to very annoying failures.

[gmaxwell]

Maybe we should have a #warn on use of 32bit field/scalar when sizeof(void*)==8, I've found some cases of people publishing benchmarks that appear to have used 32bit libsecp256k1 (while comparing to avx2 code in another library...)

[real-or-random]

#621 (comment)