Add section to show how to disable jemalloc

[johnthagen]

Closes #150

cc @RalfJung

[japaric]

Thanks, @johnthagen.

[RalfJung]

Hm, maybe the README should mention that this is abusing force_alloc_system and could stop working any time? Also see rust-lang/rust#43637 (comment).

[japaric]

Perhaps Xargo should print a warning whenever a Xargo.toml file is used? Basically, trying to build anything other than just vanilla core (and maybe vanilla std?) can break at any time due to some nightly change (crates may be renamed, the name of Cargo features may change or the features themselves may disappear, etc.).