DOC: ssl: update build instructions to use new SSL_* variables

Since commit 9a05945bd ("BUILD: add SSL_INC/SSL_LIB variables to force the
path to openssl") we have SSL_INC and SSL_LIB to point to the libssl
installation.

This commits updates the build instructions in README accordingly.
diff --git a/README b/README
index 1742680..b90e54e 100644
--- a/README
+++ b/README
@@ -97,9 +97,9 @@
     $ ./config --prefix=$STATICLIBSSL no-shared
     $ make && make install_sw
 
-When building haproxy, pass that path via ADDINC and ADDLIB to make and include
-libdl ("-ldl").
-    $ make TARGET=linux2628 USE_OPENSSL=1 ADDINC=-I$STATICLIBSSL/include ADDLIB="-L$STATICLIBSSL/lib -ldl"
+When building haproxy, pass that path via SSL_INC and SSL_LIB to make and
+include additional libs with ADDLIB if needed (in this case for example libdl):
+    $ make TARGET=linux26 USE_OPENSSL=1 SSL_INC=$STATICLIBSSL/include SSL_LIB=$STATICLIBSSL/lib ADDLIB=-ldl
 
 It is also possible to include native support for ZLIB to benefit from HTTP
 compression. For this, pass "USE_ZLIB=1" on the "make" command line and ensure