add info to README
Disable warning '-Wunused-local-typedefs'

Both Boost code and our code triggered this warning, our code
in macros that make it not quite trivial to fix.  I estimate it's
not important/useful enough a warning to keep it on.
Fix -Wconstexpr-not-const (fixes compatibility with C++1y/C++14)

"'constexpr' non-static member function will not be
 implicitly 'const' in C++1y; add 'const' to avoid
 a change in behavior"

We'd removed those consts because compilers had warned
they were redundant, but now the standard is changing
to be more sensible and the compilers are following along.

(GCC 4.8.2's libstdc++ is incompatible with C++1y's removal
of 'gets()', so I haven't yet been able to test that Lasercake
works with Clang 3.4's -std=c++1y, but at least this legitimate
warning is fixed.)