created docs.go for evidence package

evidence/doc.go

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+/*
+Package evidence handles all evidence storage and gossiping from detection to block proposal.
+For the different types of evidence refer to the `evidence.go` file in the types package.
+
+## Gossiping
+
+The core functionality begins with the evidence reactor (see reactor.
+go) which operates both the sending and receiving of evidence.
+
+The `Receive` function takes a list of evidence and does the following:
+ 1. breaks it down into individual evidence if it is `Composite Evidence` (see types/evidence.
+go#ConflictingHeadersEvidence)
+ 2. checks that it does not already have the evidence stored
+ 3. verifies the evidence against the nodes state (see state/validation.go#VerifyEvidence)
+ 4. stores the evidence to a db and a concurrent list
+
+The gossiping of evidence is initiated when a peer is added which starts a go routine to broadcast currently
+uncommitted evidence at intervals of 60 seconds (set by the by broadcastEvidenceIntervalS).
+It uses a concurrent list to store the evidence and before sending verifies that each evidence is still valid in the
+sense that it has not exceeded the max evidence age and height which should be set to be equal to the "trusting
+period" (see types/params.go#EvidenceParams).
+
+## Proposing
+
+When a new block is being proposed (in state/execution.go#CreateProposalBlock),
+`PendingEvidence(maxNum)` is called to send up to the maxNum number of uncommitted evidence, from the evidence store,
+based on a priority that is a product of the age of the evidence and the voting power of the malicious validator.
+
+Once the proposed evidence is submitted,
+the evidence is marked as committed and is moved from the broadcasted set to the committed set (
+the committed set is used to verify whether new evidence has actually already been submitted).
+As a result it is also removed from the concurrent list so that it is no longer gossiped.
+
+Last Update: 14/04/20
+*/
+package evidence