feat: add NFT mint/transfer/burn event standard

specs/Standards/NonFungibleToken/Event.md

@@ -0,0 +1,101 @@
+# Non-Fungible Token Event
+
+Version `1.0.0`
+
+## Summary
+
+Standard interfaces for NFT contract actions.
+
+## Motivation
+
+NFT-driven apps such as marketplaces regularly perform many
+of the same actions such as `minting`, `burning` and `transferring`.
+Each app have their own way of performing these actions and it
+is difficult to consistently capture these events.
+This extension addresses that.
+
+For example, it's common in NFT marketplaces to have
+different methods of transfer a single token vs many 
+tokens. Other applications like wallets, will have
+a difficult time tracking consistently this information for many
+markets. This makes interactive with many NFT-driven apps 
+infeasible.
+
+Due to discussions here 
+https://github.com/near/NEPs/issues/254,
+a standard way to capture these events is needed.
+
+## Interface
+
+Many apps may use different mechanisms to perform their batching
+requirements. This interface standardizes this process by capturing
+events of these activities through logs.
+
+```ts
+// Interface to capture an event
+// and return formatted event string.
+interface EventJsonLog {
+    // Takes `EventLogData` and returns
+    // `EVENT_JSON: <EVENT_LOG_DATA>`
+    (e:EventLogData):string
+}
+
+// Interface to capture data 
+// about an event
+// Arguments
+// * `standard`: name of standard e.g. nep171
+// * `version`: e.g. 1.0.0
+// * `event`: `nft_mint` | `nft_burn` | `nft_transfer`
+// * `data`: associate event data
+interface EventLogData {
+    standard:string,
+    version:string,
+    event:string,
+    data: NftMintLog[]|NftTransferLog[]|NftBurnLog
+}
+
+// An event log to capture token minting
+// Arguments
+// * `account_id`: "account.near"
+// * `token_id`: "1"
+interface NftMintLog {
+    owner_id:string,
+    token_id:string
+}
+
+// An event log to capture token burning
+// Arguments
+// * `owner_id`: owner of tokens to burn
+// * `token_ids`: ["1","2"]
+interface NftBurnLog {
+    owner_id:string,
+    token_ids:string[]
+}
+
+// An event log to capture token transfer
+// Arguments
+// * `sender_id`: "account.near"
+// * `receiver_id`: "receiver.near"
+// * `token_id`: "12345abc"
+interface NftTransferLog {
+    sender_id:string,
+    receiver_id:string,
+    token_id:string
+}
+```
+
+## Further methods
+
+Note that the example events covered above cover two different kinds of events:
+1. Events that are not specified in the NFT Standard (`nft_mint`, `nft_burn`)
+2. An event that is covered in the [NFT Core Standard](https://nomicon.io/Standards/NonFungibleToken/Core.html#nft-interface).
+
+ This event standard also applies beyond the three events highlighted here, where future events follow the same convention of as the second type. For instance, if an NFT contract uses the [approval management standard](https://nomicon.io/Standards/NonFungibleToken/ApprovalManagement.html), it may emit an event for `nft_approve` if that's deemed as important by the developer community.
+
+ Please feel free to open pull requests for extending the events standard detailed here as needs arise.
+## Drawbacks
+
+There is a known limitation of 16kb strings when capturing logs.
+This can be observed from `token_ids` that may vary in length
+for different apps so the amount of logs that can
+be executed may vary.