Add note explaining secure eleptic curve situation for modern TLS profile preset (#209)

h5bp/ssl/policy_modern.conf

@@ -9,10 +9,16 @@
 #     errors with old clients.
 #     If a more compatible profile is required, use intermediate policy.
 #
+# (1) The NIST curves (prime256v1, secp384r1, secp521r1) are known
+#     to be weak and potentially vulnerable but are required to support
+#     Microsoft Edge and Safari.
+#     https://safecurves.cr.yp.to/
+#
 # https://wiki.mozilla.org/Security/Server_Side_TLS#Recommended_configurations
 # https://nginx.org/en/docs/http/ngx_http_ssl_module.html
 
 ssl_protocols TLSv1.2;
 ssl_ciphers EECDH+CHACHA20:EECDH+AES;
+# (1)
 ssl_ecdh_curve X25519:prime256v1:secp384r1:secp521r1;
 ssl_prefer_server_ciphers on;