[readme] Update README with short repo explanation and link to guidelines
[readme] Update README with short repo explanation and link to guidelines

exponentjs/exponent,exponentjs/exponent,exponentjs/exponent,exponent/exponent,exponent/exponent,exponent/exponent,exponentjs/exponent,exponentjs/exponent,exponent/exponent,exponentjs/exponent,exponent/exponent,exponentjs/exponent,exponent/exponent,exponentjs/exponent,exponentjs/exponent,exponent/exponent,exponent/exponent,exponent/exponent