Add a note about assembly test suite.

Author: denzp

src/tests/intro.md

@@ -38,6 +38,8 @@ that give more details.
 - `debuginfo` – tests that run in gdb or lldb and query the debug info
 - `codegen` – tests that compile and then test the generated LLVM
   code to make sure that the optimizations we want are taking effect.
+- `assembly` – similar to `codegen` tests, but verifies assembly output
+  to make sure LLVM target backend can handle provided code.
 - `mir-opt` – tests that check parts of the generated MIR to make
   sure we are building things correctly or doing the optimizations we
   expect.