Update README.md

Author: ciquLi

README.md

@@ -14,8 +14,12 @@ Steps:
 2. Download pfx2as file from CAIDA
 3. Generate the mapping db file
 4. Usage
+
   import pyasn
+  
   asndb = pyasn.pyasn('ipasn.dat')
+  
   asndb.lookup('8.8.8.8')  # should return: (15169, '8.8.8.0/24'), the origin AS, and the BGP prefix it matches
+  
   asndb.get_as_prefixes(1128) # returns ['130.161.0.0/16', '131.180.0.0/16', '145.94.0.0/16'], TU-Delft prefixes