Add mender signing key doc

docs/README-mender-keys

@@ -0,0 +1,27 @@
+$ openssl ecparam -genkey -name prime256v1 -out private-and-params.key
+
+$ openssl ec -in private-and-params.key -out private.key
+read EC key
+writing EC key
+
+$ openssl ec -in private-and-params.key -pubout -out public.key
+read EC key
+writing EC key
+
+$ ls -l
+total 12
+-rw-rw-r-- 1 scott scott 302 Dec 28 14:11 private-and-params.key
+-rw-rw-r-- 1 scott scott 227 Dec 28 14:12 private.key
+-rw-rw-r-- 1 scott scott 178 Dec 28 14:12 public.key
+
+The public key gets copied to
+
+  meta-bbb/recipes-mender/mender/files/artifact-verify-key.pem
+
+to be installed on the devices.
+
+The private key is used to sign the artifacts with the mender-artifact tool.
+
+See the script
+
+  meta-bbb/scripts/sign-mender-image.sh