Missing note on Boost.Build configuration

[mloskot]

In the README.md, it is clearly stated the build configuration assumes either use of user-config.jam file or BOOST_ROOT env var.

For the sake of completeness, it may be worth adding similar note about boost-build.jam, which is required if the env var is not used.
It may help newcomers to miss the problem of

Unable to load Boost.Build: could not find "boost-build.jam"