[introduction] Add a introduction starter

We describe main ideas of DLC, therefore oracle, DLC, offers negotiation
contract execution transaction, adaptor sigs.

Introduction.md

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+# Introduction and Index
+
+These standards documents describe a framework for onchain/off-chain stateful
+bitcoin contracts, relying on real-world obversations from third-party, without
+leaking knowledge of usage. Bitcoin contracts are cooperatively built and signed
+by every party involved.
+
+## The Bet: A Short Introduction to Discreet Log Contracts
+
+Discreet Log Contracts is a set of formats to get data external to the bitcoin
+system into the contract.
+
+### Oracle
+
+An oracle is an entity signing messages in reaction to real-world events. Before
+event occurence they commit a public key on any publication space and the set of
+messages mapping to event properties. At occurence they release a signed message
+corresponding to the specific instance of the event.
+
+Currently, this protocol version doesn't support any mitigations against oracle
+equivocation, beyond confidentiality of oracle message usage by parties.
+
+### DLC contract
+
+A description of a set of potential oracles messages mapped to bitcoin balances,
+negotiated by parties. Among all potential balances, only one can be finalized.
+
+In case of lazy oracle, parties may abort contract and refund their contributions.
+
+In case of cheating counter-party, i.e broadcasting a state for which oracle
+scalar isn't known, the honest protocol participant can claims all the balance.
+
+### Contract Execution Transaction
+
+XXX
+
+### Offers negotiation
+
+XXX
+
+
+## Glossary and Terminology Guide
+
+XXX: set of transactions
+XXX: peers
+XXX: oracles
+XXX: oracles signatures