add migration.md to track the process

migration.md

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+# Edgeware Migration
+Based on [PRs with migrations hackMD](https://hackmd.io/p97ldDxfST6WF-1m-MxwOA).
+
+## Migration PRs
++ [unique storage names](https://github.com/paritytech/substrate/pull/5010/)
++ [iterable storage maps (away from opaque hashes)](https://github.com/paritytech/substrate/pull/5226)
++ [democracy redesign](https://github.com/paritytech/substrate/pull/5294/)
++ [prime member selection](https://github.com/paritytech/substrate/pull/5346)
+
+### Removed Migrations
++ https://github.com/paritytech/substrate/pull/5870
++ https://github.com/paritytech/substrate/pull/5291
++ https://github.com/paritytech/substrate/pull/5224
+
+## Pallets
++ System: frame_system
++ Utility: pallet_utility
++ Aura: pallet_aura
+
++ Timestamp: pallet_timestamp
++ Authorship: pallet_authorship
++ Indices: pallet_indices
++ Balances: pallet_balances
++ TransactionPayment: pallet_transaction_payment
+
++ Staking: pallet_staking
++ Session: pallet_session
++ Democracy: pallet_democracy
++ Council: pallet_collective::<Instance1>
++ Elections: pallet_elections_phragmen
+
++ FinalityTracker: pallet_finality_tracker
++ Grandpa: pallet_grandpa
++ Treasury: pallet_treasury
++ Contracts: pallet_contracts
+
++ Sudo: pallet_sudo
++ ImOnline: pallet_im_online
++ AuthorityDiscovery: pallet_authority_discovery
++ Offences: pallet_offences
++ RandomnessCollectiveFlip: pallet_randomness_collective_flip
++ Identity: pallet_identity
++ Scheduler: pallet_scheduler
++ Vesting: pallet_vesting
++ EVM: pallet_evm
++ Historical: pallet_session_historical
+
++ Signaling: signaling
++ Voting: voting
++ TreasuryReward: treasury_reward