Include "ba" targets in opam build

[ghost]

The "ba" targets have been added to the opam build. This makes it easy to use the current "master" branch in a build, for when you also need the BigArray support.

Added a line to the README.md detailing how the current development branch can be used without manually getting and installing.

[csoren]

This is fine with me

[fccm2]

I'm not sure if the command line to compile with bigarrays is the same for all ocaml versions.
This is why I didn't include it by default.

[fccm2]

chatgpt tells me that the command line changes with ocaml 4.06 (to compile with ba's).
Before this version, it seems that no specific parameters was needed to compile with bigarrays, but had to be added with 4.06 and after.
I don't know if this additional parameter would produce an error with versions before 4.06 (or just be ignored).
I also don't know how to add a version switch in the opam file, to change the command line to compile with all past ocaml versions.
chatgpt tells me that it's possible to add a version switch in the build section of the opam file like this:

build: [
  ["compile with ba parameters"] {ocaml:version >= "4.06"}
  ["compile without ba parameters"] {ocaml:version < "4.06"}
]

PS: please also notice, that the functions wrapped with a ba data structure, are also available with a string/bytes data structure, that you can use with the new functions from the String module, to read/write ints from/to a string/bytes.