Add basic documentation for the profile feature

Author: marco-c

src/doc/unstable-book/src/compiler-flags/profile.md

@@ -3,3 +3,19 @@
 The tracking issue for this feature is: None
 
 ------------------------
+
+This feature allows the generation of code coverage reports.
+
+Set the compiler flags `-Ccodegen-units=1 -Clink-dead-code -Cpasses=insert-gcov-profiling -Zno-landing-pads` to enable gcov profiling.
+
+For example:
+```Bash
+cargo new testgcov --bin
+cd testgcov
+export RUSTFLAGS="-Ccodegen-units=1 -Clink-dead-code -Cpasses=insert-gcov-profiling -Zno-landing-pads"
+cargo build
+cargo run
+```
+
+Once you've built and run your program, files with the `gcno` (after build) and `gcda` (after execution) extensions will be created.
+You can parse them with [llvm-cov gcov](http://llvm.org/docs/CommandGuide/llvm-cov.html#llvm-cov-gcov) or [grcov](https://github.com/marco-c/grcov).