add some placeholder text, short intro

Author: mark-i-m

src/backend/backend.md

@@ -1 +1,17 @@
 # The Compiler Backend
+
+The _compiler backend_ refers to the parts of the compiler that turn rustc's
+MIR into actual executable code (e.g. an ELF or EXE binary) that can run on a
+processor. This is the last stage of compilation, and it has a few important
+parts:
+
+0. First, we need to collect the set of things to generate code for. In
+   particular, we need to find out which concrete types to substitute for
+   generic ones, since we need to generate code for the concrete types.
+   Generating code for the concrete types (i.e. emitting a copy of the code for
+   each concrete type) is called _monomorphization_, so the process of
+   collecting all the concrete types is called _monomorphization collection_.
+1. Next, we need to actually lower the MIR (which is generic) to a codegen IR
+   (usually LLVM IR; which is not generic) for each concrete type we collected.
+2. Finally, we need to invoke LLVM, which runs a bunch of optimization passes,
+   generates executable code, and links together an executable binary.

src/backend/lowering-mir.md

@@ -0,0 +1,3 @@
+# Lowering MIR to a Codegen IR
+
+TODO

src/backend/monomorph.md

@@ -1 +1,8 @@
 # Monomorphization
+
+TODO
+
+
+## Polymorphization
+
+TODO