doc: update README with build instruction

[Shadow53]

A couple things to note:

• I did my best to manually validate that these commands are correct
• That said, when I tried to automatically verify using a Dockerfile, I ran into errors when compiling libquilc.so because gcc -dynamiclib is not a valid flag:

#0 34.66 * gcc -dynamiclib -o libquilc.dylib libquilc.c -lsbcl
#0 34.69 cc1: warning: unrecognized gcc debugging option: y
#0 34.69 cc1: warning: unrecognized gcc debugging option: n
#0 34.69 cc1: warning: unrecognized gcc debugging option: m
#0 34.69 cc1: warning: unrecognized gcc debugging option: i
#0 34.69 cc1: warning: unrecognized gcc debugging option: c
#0 34.69 cc1: warning: unrecognized gcc debugging option: l
#0 34.69 cc1: warning: unrecognized gcc debugging option: i
#0 34.69 cc1: warning: unrecognized gcc debugging option: b

[MarquessV]

RE: the gcc issue. -dynamiclib is a darwin only flag.

[MarquessV]

I think this is worth a ✅ from Mark given he's the one who knows the most about it, but these directions align with my notes and state of my machine.

[notmgsk]

Just a couple comments. Maybe something to do in a follow-on issue would be to collect those build instructions into a Dockerfile (or similar).

I could see it being useful for us to pin the various quil-lang repos to particular revs, so that our instructions are never out-of-date.