Update README.md

[DarioHett]

I ran intho the following issue trying to hack on the implementation.

>> opam --version
2.1.5
>> opam switch create 5.0.0+trunk --repos alpha,default

<><> Installing new switch packages <><><><><><><><><><><><><><><><><><><><><><>
Switch invariant: ["ocaml-variants" {= "5.0.0+trunk"}]
[ERROR] Could not determine which packages to install for this switch:
  * Missing dependency:
    - ocaml-variants > 5.3.1~
    no matching version
>> opam switch list-available --repositories alpha,beta,default
ocaml-base-compiler                    5.0.0                                  Official release 5.0.0
ocaml-variants                         5.0.0+options                          Official release of OCaml 5.0.0
ocaml-variants                         5.0.0+tsan                             OCaml 5.0.0, with ThreadSanitizer instrumentation
ocaml-variants                         5.0.1+trunk                            Latest 5.0 development
ocaml-base-compiler                    5.1.0~alpha1                           First alpha release of OCaml 5.1.0
>> opam switch create 5.0.1+trunk --repos alpha,default

<><> Installing new switch packages <><><><><><><><><><><><><><><><><><><><><><>
Switch invariant: ["ocaml-variants" {= "5.0.1+trunk"}]

<><> Processing actions <><><><><><><><><><><><><><><><><><><><><><><><><><><><>
∗ installed base-bigarray.base
∗ installed base-threads.base
∗ installed base-unix.base
⬇ retrieved ocaml-variants.5.0.1+trunk  (https://github.com/ocaml/ocaml/archive/5.0.tar.gz)
∗ installed ocaml-variants.5.0.1+trunk
∗ installed ocaml-config.3
∗ installed ocaml.5.0.1
∗ installed base-domains.base
∗ installed base-nnp.base
Done.
# Run eval $(opam env --switch=5.0.1+trunk) to update the current shell environment`

[polytypic]

Thanks!

That opam switch create ... instruction must have been written a long time ago and no longer makes sense.

We should recommend using the latest version of the OCaml compiler, which, at the moment, is 5.1.1 so the command to use today would be opam switch create 5.1.1.

However, I would recommend avoiding specifying a compiler version like that in the README. OCaml 5.1.1 is also known to have multicore related bugs and issues and will be outdated soon.

We should instead recommend using the latest official release whose version is greater than 5.0.0. You can use opam switch list-available to see the list of available versions.