supervisor: Add an example on the HSM extension

The HSM extension from SBI v2.0 allows a supervisor to manage the state
of harts on the system. Moreover, it also guarantees that only one hart
makes it into the booting stages of the supervisor, and it's up to the
supervisor to wake up the rest through the HSM extension.

I have added an example which showcases this extension in different
ways, and it finally shuts down the machine.

Signed-off-by: Miquel Sabaté Solà <mikisabate@gmail.com>

1 files changed, 1 insertions, 1 deletions

diff --git a/README.md b/README.md
index a31a3f5..00e8851 100644
--- a/README.md
+++ b/README.md
@@ -1,4 +1,4 @@
-A bunch of code to messes with low level stuff. That is:
+A bunch of code that messes with low level stuff. That is:
 
 - `arch`:
   - `machine`: prints "hello, world" on the serial port while running in M mode