Warn about backtrace recording in README

[mpu]

I was thinking my old laptop really needed to be replaced as it took it more than 20 minutes to load HOL Light. Some investigation revealed that all the time was spent recording backtraces. Disabling the recording brought down the loading time to 2 minutes.

This PR proposes to add a warning in the README about this potential issue. I will let you judge if it belongs where I inserted it.

Thanks!

[jrh13]

Very interesting point, something I'd never been aware of.
I'm happy indeed to add a warning to the README. Do you
know any more about the circumstances that in general
lead to the backtrace recording being on by default? I've
not noticed this happening in my own OCaml instances,
which are mainly from the debian or homebrew package
managers.

[mpu]

The problem might not happen to many, so I now think that
adding a step like I did in the README might be a bit overkill.

The reason it hit me is that I had OCAMLRUNPARAM=b set
in my shell configuration (it is handy when doing ocaml
development to print backtraces).

It looks like Jane Street's Base library (a popular stdlib
replacement) also has recording on by default.

[mpu]

I'm going to close the PR. If someone has perf problems
maybe Google will be able to help! At least, now you are
also aware of this potential problem.