Added details about additional options for saithrift generation.

This is follow-up work from opencomputeproject#1533.

Signed-off-by: Chris Sommers <chrispsommers@gmail.com>

ptf/SAI_PTF_user-guide.md

@@ -81,6 +81,29 @@ Build SAI thrift server and libraries
 
 ```bash
 export SAITHRIFTV2=y
+export GEN_SAIRPC_OPTS="-ve" # optional - to use SAI experimental headers
+# Below is an extended example of linking additional application-specific libraries besides libsai.
+# Modify to suit a particular use-case. Omit if not needed.
+export SAIRPC_EXTRA_LIBS="\
+		-L/lib/x86_64-linux-gnu -Wl,-rpath=/lib/x86_64-linux-gnu -lm \
+		-L/usr/local/lib/ -Wl,-rpath=/usr/local/lib \
+	    -lpthread \
+	    -lpiprotogrpc \
+	    -lpiprotobuf \
+	    -lprotobuf \
+	    -lgrpc++ \
+	    -lgrpc \
+	    -lpiall \
+	    -lpi_dummy \
+	    -lpthread \
+	    -labsl_synchronization \
+	    -labsl_status \
+		-labsl_raw_hash_set \
+		-lgpr \
+		-lre2 \
+		-lssl \
+		-laddress_sorting"
+
 make saithrift-build
 make saithrift-install
 ```