Clarify in docs that SEAL_BUILD_SEAL_C builds shared library by default

[sloede]

I tried for a while to build the C interface as a shared library, providing -DSEAL_BUILD_SEAL_C=ON -DBUILD_SHARED_LIBS=ON, only to get an error message from CMake that BUILD_SHARED_LIBS is not allowed when building the C interface:

SEAL/CMakeLists.txt

Lines 437 to 439 in 5c586e2

	if(BUILD_SHARED_LIBS)
	message(FATAL_ERROR "SEAL_C requires `BUILD_SHARED_LIBS=OFF`")
	endif()

Almost ready to give up, I thankfully realized that by default the C interface is already built as a shared library 🙈 To spare future users the same mistakes I made, I suggest to clarify in the README that when building the C interface, a shared library is created by default.

[kimlaine]

Thanks for pointing this out; sorry it was unclear. We'll address this in the next release.