Add some building documentation that clarifies the make targets

doc/BUILDING.md

@@ -0,0 +1,29 @@
+# Building
+
+To build oas in the default configuration you just need (gnu) make and a
+sufficiently modern clang.
+
+```
+make
+```
+
+## Make targets
+
+There are a number of make targets available to build various instrumented
+builds that are used in validation, analysis and sanitizing. Some of these may
+require extra dependencies.
+
+
+ - `debug`: Creates the debug build in `build/debug`. This is the default target.
+ - `all`: Builds all binary executable targets. These are
+   `debug`, `release`, `msan`, `asan` and `afl`. All executables can be found
+   in `build/` in a subdirectory matching their target names.
+ - `release`: Creates the release build in `build/release`
+ - `afl`: Creates a build with AFL++ instrumentation for fuzzing
+ - `fuzz`: Starts the fuzzer with the instrumented afl executable
+ - `asan`: builds with the address and undefined clang sanitizers
+ - `msan`: builds with the memory clang sanitizer
+ - `validate`: Builds `debug`, `msan`, and `asan` targets, then runs the
+   validation script. This script executes the sanitizer targets and runs
+   Valgrind on the debug target across multiple modes and test input files.
+