update readme

README.md

@@ -19,5 +19,22 @@ $ shuffle3 -u file
 ## Other options
 Run with `--help` for more options.
 
+# Building
+Run `make` to build the normal binary. It will output to `shuffle3-release`.
+
+## Release target
+The `release` (default) target uses the variables `RELEASE_CFLAGS` and `RELEASE_LDFLAGS` to specify opitimisations. These can be set by you if you wish.
+
+### Note
+The default `RELEASE_CFLAGS` contains the flag `-march=native`. This may be underisable for you, in which case set the variable or modify the makefile to remove it.
+
+## Debug target
+To build with debug information, run `make debug`. Extra debug flags can be provided with the `DEBUG_CFLAGS` and `DEBUG_LDFLAGS` variables which have default values in the Makefile.
+
+The build and unstripped binary will be `shuffle3-debug`.
+
+## Notes
+Before switching between `release` and `debug` targets, remember to run `make clean`.
+
 # License
 GPL'd with <3