add Transaction Malleability (#150)

* add Transaction Malleability

* fix https://github.com/lnbook/lnbook/pull/150#discussion_r379919498

* explain why non-malleability is needed

* fix typos

glossary.asciidoc

@@ -448,7 +448,10 @@ Transaction::
     Transactions can also contain contracts and should not be confused with payments.
 
 Transaction Malleability::
-    TBD.
+    Transaction Malleability is a property that hash of a transaction can change without changing the semantic of the transaction (the UTXOs it is spending, the destinations and the corresponding amounts).
+    For example, altering the signature can change the hash of a transaction, because of the non-deterministism of ECDSA signing.
+    A Commitment Transaction needs the hash of a Funding Transaction, if the hash of the Funding Transaction changes, transactions depending on it will become invalid. This will make users unable to claim the refunds if there is.
+    Segregated Witness soft fork addresses this issue and therefore is an important upgrade to support Lightning network.
 
 Transport Layer::
     TBD.