doc: Added default signet config for linearize script

contrib/linearize/example-linearize.cfg

@@ -13,6 +13,9 @@ port=8332
 #regtest default
 #port=18443
 
+#signet default
+#port=38332
+
 # bootstrap.dat hashlist settings (linearize-hashes)
 max_height=313000
 
@@ -33,6 +36,11 @@ input=/home/example/.bitcoin/blocks
 #genesis=0f9188f13cb7b2c71f2a335e3a4fc328bf5beb436012afca590b1a11466e2206
 #input=/home/example/.bitcoin/regtest/blocks
 
+# signet
+#netmagic=0a03cf40
+#genesis=00000008819873e925422c1ff0f99f7cc9bbb232af63a077a480a3633bee1ef6
+#input=/home/example/.bitcoin/signet/blocks
+
 # "output" option causes blockchain files to be written to the given location,
 # with "output_file" ignored. If not used, "output_file" is used instead.
 # output=/home/example/blockchain_directory