Update README.md

Author: parulnith

README.md

@@ -33,7 +33,15 @@ For platform-specific instructions, read [here](https://www.nltk.org/install.htm
 ```
 pip install nltk
 ```
-
+### Installing required packages
+After NLTK has been downloaded, install required packages
+```
+import nltk
+from nltk.stem import WordNetLemmatizer
+nltk.download('popular', quiet=True) # for downloading popular packages
+nltk.download('punkt') 
+nltk.download('wordnet') 
+```