Write down the requirement for the Zaamo extension

Since 2eead36 ("Run a hart lottery on SMP") the kernel is making
use of the 'amoadd.w' instruction, which requires the Zaamo
extension (included in the base A extension). This is notable and is
worth a mention on the list of requirements.

Signed-off-by: Miquel Sabaté Solà <[email protected]>

README.md

@@ -134,6 +134,8 @@ things simple. Hence, here there are some limitations/requirements:
   target, and that's what you are supposed to be passing to the kernel.
 - RISC-V is ambivalent on the endianness. Not this kernel. Out of simplicity we
   require a little-endian RISC-V system.
+- This kernel makes use of the `Zaamo` Extension for Atomic Memory Operations,
+  which is included under the `A` Extension for Atomic Instructions.
 
 ## Special thanks to
 

include/fbos/init.h

@@ -11,7 +11,7 @@
 // This greatly simplifies the specification and the hardware, but for the
 // kernel this means that harts will appear randomly. In order to know which
 // hart runs first, in Linux they run a "lottery": an atomic value holds how
-// many harts hav already been seen. The first hart to appear will actually
+// many harts have already been seen. The first hart to appear will actually
 // initialize things before bringing the others up, while the others will simply
 // wait until the first hart frees the lock for them.
 //