overview and installation (from DEN)

docs/installation.rst

@@ -1,2 +1,12 @@
 Install PyTeal
 ==============
+
+The easiest way of installing PyTeal is using :code:`pip` : ::
+
+  pip install pyteal
+
+You can also install from source:
+
+  TODO: instructions for install from source
+
+

docs/overview.rst

@@ -1,2 +1,74 @@
 Overview
 ========
+
+With PyTeal, developers can easily write `Algorand Smart Contracts (ASC1s) <https://developer.algorand.org/docs/asc>`_ in Python.
+
+Below is the example of writing *Hashed Time Locked Contract* in Pyteal::
+
+  from pyteal import *
+
+  """ Hash Time Locked Contract
+  """
+  alice = Addr("6ZHGHH5Z5CTPCF5WCESXMGRSVK7QJETR63M3NY5FJCUYDHO57VTCMJOBGY")
+  bob = Addr("7Z5PWO2C6LFNQFGHWKSK5H47IQP5OJW2M3HA2QPXTY3WTNP5NU2MHBW27M")
+  secret = Bytes("base32", "23232323232323")
+
+  fee_cond = Txn.fee() < Int(1000)
+  
+  type_cond = Txn.type_enum() == Int(1)
+  
+  recv_cond = And(Txn.close_remainder_to() == Global.zero_address(),
+                  Txn.receiver() == alice,
+                  Sha256(Arg(0)) == secret)
+		  
+  esc_cond = And(Txn.close_remainder_to()  == Global.zero_address(),
+                 Txn.receiver() == bob,
+                 Txn.first_valid() > Int(3000))
+
+  atomic_swap = And(fee_cond,
+                    type_cond,
+		    Or(recv_cond, esc_cond))
+
+  print(atomic_swap.teal())
+
+
+As shown in this exmaple, the logic of smart contract is expressed using PyTeal expressions constructed in Python. PyTeal overloads Python's arithmetic operators 
+such as :code:`<` and :code:`==` (more overloaded operators can be found in TODO), allowing Python developers express smart contract logic more naturally.
+
+Last, :code:`teal()` is called to convert an PyTeal expression
+to a TEAL program, consisting a sequence of TEAL opcodes.
+The output of the above example is: ::
+
+  txn Fee
+  int 1000
+  <
+  txn TypeEnum
+  int 1
+  ==
+  &&
+  txn CloseRemainderTo
+  global ZeroAddress
+  ==
+  txn Receiver
+  addr 6ZHGHH5Z5CTPCF5WCESXMGRSVK7QJETR63M3NY5FJCUYDHO57VTCMJOBGY
+  ==
+  &&
+  arg 0
+  sha256
+  byte base32 23232323232323
+  ==
+  &&
+  txn CloseRemainderTo
+  global ZeroAddress
+  ==
+  txn Receiver
+  addr 7Z5PWO2C6LFNQFGHWKSK5H47IQP5OJW2M3HA2QPXTY3WTNP5NU2MHBW27M
+  ==
+  &&
+  txn FirstValid
+  int 3000
+  >
+  &&
+  ||
+  &&
+