cmd/compile: reorganise and improve ssa/README.md

Since the initial version was written, I've gotten help writing
cmd/compile/README.md and I've also learned some more on my own, so it's
time to organise this document better and expand it.

First, split up the document in sections, starting from the simplest
ideas that can be explained on their own. From there, build all the way
up into SSA functions and how they are compiled.

Each of the sections also gets more detail now; most ideas that were a
paragraph are now a section with several paragraphs. No new major
sections have been added in this CL.

While at it, add a copyright notice and make better use of markdown,
just like in the other README.md.

Also fix a file path in value.go, which I noticed to be stale while
reading godocs to write the document.

Finally, leave a few TODO comments for areas that would benefit from
extra input from people familiar with the SSA package. They will be
taken care of in future CLs.

Change-Id: I85e7a69a0b3260e72139991a625d926099624f71
Reviewed-on: https://go-review.googlesource.com/110067Reviewed-by: Keith Randall <khr@golang.org>

src/cmd/compile/internal/ssa/value.go

@@ -25,7 +25,7 @@ type Value struct {
 	Op Op
 
 	// The type of this value. Normally this will be a Go type, but there
-	// are a few other pseudo-types, see type.go.
+	// are a few other pseudo-types, see ../types/type.go.
 	Type *types.Type
 
 	// Auxiliary info for this value. The type of this information depends on the opcode and type.