README: update toolchain recommendations

This hadn't been touched in a while, and we now need gcc6+

Fixes: https://github.com/libopencm3/libopencm3/issues/1459
Signed-off-by: Karl Palsson <karlp@tweak.au>

README.md

@@ -80,15 +80,15 @@ After that you can navigate to the folder where you've extracted libopencm3 and
 Toolchain
 ---------
 
-The most heavily tested toolchain is "gcc-arm-embedded"
-https://launchpad.net/gcc-arm-embedded
+The most heavily tested toolchain is ["gcc-arm-embedded"](https://developer.arm.com/Tools%20and%20Software/GNU%20Toolchain)
+This used to be available at https://launchpad.net/gcc-arm-embedded
 
 Other toolchains _should_ work, but they have not been nearly as well tested.
 Toolchains targeting Linux, such as "gcc-arm-linux-gnu" or the like are
 _not_ appropriate.
 
-_NOTE_: We recommend that you use gcc-arm-embedded version 4.8 2014q3 or newer
-to build all platforms covered by libopencm3 successfully.
+_NOTE_: GCC version 6 or later is required, as we're using attributes on enumerators
+to help mark deprecations.
 
 Building
 --------