Document the possibility for command line argument env assignments

Reviewed-by: NAndy Polyakov <appro@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/4818)

CHANGES

@@ -9,6 +9,10 @@
 
  Changes between 1.1.0f and 1.1.1 [xx XXX xxxx]
 
+  *) Make it possible to have environment variable assignments as
+     arguments to config / Configure.
+     [Richard Levitte]
+
   *) Add multi-prime RSA (RFC 8017) support.
      [Paul Yang]
 

INSTALL

@@ -532,6 +532,14 @@
                    passed through as they are to the compiler as well.  Again,
                    consult your compiler documentation.
 
+  VAR=value
+                   Assignment if environment variable for Configure.  These
+                   work just like normal environment variable assignments,
+                   but are supported on all platforms and are confined to
+                   the configuration scripts only.  These assignments override
+                   the corresponding value in the inherited environment, if
+                   there is one.
+
 
  Installation in Detail
  ----------------------