Fix lib links in README.rst (aws#79)

The new links resolve to readthedocs. Previously, the links would follow pages
that didn't exist. This allows users to read documentation referenced in the
README by clicking on the links.

README.rst

@@ -178,9 +178,9 @@ of the one that the client would normally construct for you.
 .. _cryptography: https://cryptography.io/en/latest/
 .. _cryptography installation guide: https://cryptography.io/en/latest/installation/
 .. _boto3: https://boto3.readthedocs.io/en/latest/
-.. _EncryptedClient: lib/encrypted/client.html
-.. _EncryptedResource: lib/encrypted/resource.html
-.. _EncryptedTable: lib/encrypted/table.html
-.. _CryptoConfig: lib/encrypted/config.html
-.. _decrypt_dynamodb_item: lib/encrypted/item.html#dynamodb_encryption_sdk.encrypted.item.decrypt_dynamodb_item
-.. _transformation functions: lib/tools/transform.html
+.. _EncryptedClient: https://aws-dynamodb-encryption-python.readthedocs.io/en/latest/lib/encrypted/client.html
+.. _EncryptedResource: https://aws-dynamodb-encryption-python.readthedocs.io/en/latest/lib/encrypted/resource.html
+.. _EncryptedTable: https://aws-dynamodb-encryption-python.readthedocs.io/en/latest/lib/encrypted/table.html
+.. _CryptoConfig: https://aws-dynamodb-encryption-python.readthedocs.io/en/latest/lib/encrypted/config.html
+.. _decrypt_dynamodb_item: https://aws-dynamodb-encryption-python.readthedocs.io/en/latest/lib/encrypted/item.html#dynamodb_encryption_sdk.encrypted.item.decrypt_dynamodb_item
+.. _transformation functions: https://aws-dynamodb-encryption-python.readthedocs.io/en/latest/lib/tools/transform.html