Just a little clean-up of the README

[didriklundberg]

No description provided.

[palmskog]

@didriklundberg isn't this a good time to skip the instructions about opam? If p4Script.sml is in version control, most people probably do not want to bother with Ott. There could be some separate instructions to "regenerate Ott file" or similar, unrelated to INSTALL.md.

[didriklundberg]

@didriklundberg isn't this a good time to skip the instructions about opam? If p4Script.sml is in version control, most people probably do not want to bother with Ott. There could be some separate instructions to "regenerate Ott file" or similar, unrelated to INSTALL.md.

opam is needed for petr4, which we need in order to import .p4 programs.

Do you think that the current arrangement of Ott installation in an optional step is fine, or should it fall under a separate subsection?