update readme

README.md

@@ -1,6 +1,6 @@
 # fcmp - compare file identity
 
-A shell utils that compares files for identity with `mmap()` and `memcmp()`.
+A small shell util that compares files for identity with `mmap()` and `memcmp()`.
 
 # Usage
 `fcmp` produces no output, but success is indicated by its return code.
@@ -16,7 +16,7 @@ $ fcmp file1 file2 && echo "Equal!"
 | 2    | The files have unequal lengths, and therefor must be unequal |
 | -1   | There was an error                                           |
 
-Therefor you can use this in shell scripts easily:
+You can use this in shell scripts easily:
 
 ``` shell
 # Example
@@ -28,5 +28,21 @@ else
 fi
 ```
 
+# Building
+To build normally, run `make`.
+
+## Release target 
+Build with default optimisations using `make release`, it will output a stripped binary at `fcmp-release`.
+
+### Notes
+* The Makefile uses variables `RELEASE_CFLAGS` and `RELEASE_LDFLAGS` to apply optimisations (and `DEBUG_CFLAGS` + `DEBUG_LDFLAGS` for extra compiler flags with the debug target). If needed you can set these yourself to prevent the defaults.
+* The default `RELEASE_CFLAGS` specify `-march=native` which may be undesireable for you. Set the variable or modify the Makefile if you need to remove this.
+
+## Debug target
+Build with debugging information and no optimisations using `make debug`, it will output a binary at `fcmp-debug`.
+
+## Note
+Before switching between `release` and `debug` targets, make sure to run `make clean`.
+
 # License
 GPL'd with <3