Document JEMALLOC_OVERRIDE

This adds a README paragraph explaining how to provider a prebuilt
version of malloc through `JEMALLOC_OVERRIDE`, as well as build flags
expected by `tikv-jemalloc-sys`.

Author: nmattia

jemalloc-sys/README.md

@@ -150,6 +150,14 @@ hyphens `-` are replaced with underscores `_`(see
   virtual address size on those platforms where it knows how, and picks a
   default otherwise. This option may be useful when cross-compiling.
 
+By default this crate builds and links against a vendored version of `jemalloc`. To
+provide your own `jemalloc` version, set `JEMALLOC_OVERRIDE` to point to a built
+version of `jemalloc`. This can point to either a shared or static library. For
+instance: `JEMALLOC_OVERRIDE=/path/to/libjemalloc.a`. The library should be built
+with `jemalloc`'s `--with-jemalloc-prefix=_rjem_`. For static libraries it is also
+recommended to build with `--with-private-namespace=_rjem_`. For more information
+see [`jemalloc/INSTALL.md`][jemalloc_install].
+
 [jemalloc_install]: https://github.com/jemalloc/jemalloc/blob/dev/INSTALL.md#advanced-configuration
 
 ## License