Rename cakey.pem to cacert.key for consistency

And add a README with some basic instructions for newcomers.
diff --git a/tools/certs/README.md b/tools/certs/README.md
new file mode 100644
index 0000000..b00b7ab
--- /dev/null
+++ b/tools/certs/README.md
@@ -0,0 +1,2 @@
+To enable https://web-platform.test:8443/, add cacert.pem to your browser as Certificate Authority.
+E.g., for Firefox, go to about:preferences and search for "certificates".
diff --git a/tools/certs/cakey.pem b/tools/certs/cacert.key
similarity index 100%
rename from tools/certs/cakey.pem
rename to tools/certs/cacert.key
diff --git a/tools/sslutils/openssl.py b/tools/sslutils/openssl.py
index 09c2471..f44d0de 100644
--- a/tools/sslutils/openssl.py
+++ b/tools/sslutils/openssl.py
@@ -136,7 +136,7 @@
 new_certs_dir = $certs
 crl_dir = $dir%(sep)scrl
 database = $dir%(sep)sindex.txt
-private_key = $dir%(sep)scakey.pem
+private_key = $dir%(sep)scacert.key
 certificate = $dir%(sep)scacert.pem
 serial = $dir%(sep)sserial
 crldir = $dir%(sep)scrl
@@ -294,7 +294,7 @@
         return self._ca_cert_path
 
     def _load_ca_cert(self):
-        key_path = self.path("cakey.pem")
+        key_path = self.path("cacert.key")
         cert_path = self.path("cacert.pem")
 
         if self.check_key_cert(key_path, cert_path, None):
@@ -327,7 +327,7 @@
         path = self.path
         self.logger.info("Generating new CA in %s" % self.base_path)
 
-        key_path = path("cakey.pem")
+        key_path = path("cacert.key")
         req_path = path("careq.pem")
         cert_path = path("cacert.pem")