note libtls dependency for HTTPS constraint

Document CA path option.

INSTALL

@@ -98,6 +98,10 @@ provided by autoconf itself:
 	is "_ntp". The home directory of this user will be used for privilege
 	separation.
 
+--with-cacert=[path]
+	Specify the CA certificate location for HTTPS constraint validation.
+	Defaults to /etc/ssl/certs/ca-certificates.crt
+
 If you need to pass special options to the compiler or linker, you
 can specify these as environment variables before running ./configure.
 For example:

README

@@ -13,6 +13,8 @@ Platform Requirements
  - daemon(3), setresgid(2), setresuid(2) or equivalent
  - A working arc4random(2) implementation, OpenSSL or LibreSSL
      (this project shares the same arc4random compatibility code from LibreSSL)
+ - libtls (included with LibreSSL 2.1.4 or higher) is required for HTTPS time
+   constraint validation.
 
 At the time of writing the Portable version is known to build and work on: