Add initial design doc and readme

.gitignore

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+# Vim 
+*.swp
+
+# Binary
+rekorsidekick

README.md

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+# 🔍 Rekor Sidekick
+
+Rekor Sidekick monitors a Rekor signature transparency log and forwards
+events of interest where ever you like.
+
+> NB: 🛠️ This software is prerelease!

docs/design.md

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+# Rekor Sidekick design
+
+The basic design of Rekor sidekick is to continually pull entries from
+a Rekor server, check if the entries are of interest using a configured set
+of policies and to forward entries of interest a configured set of outputs
+
+```
+                               ┌─────────────────┐
+                               │  Event Policies │
+                               └──────┬───▲──────┘
+                                      │   │
+                             Decision │   │ Should forward entry?
+                                      │   │
+                                      │   │
+                                      │   │                           Outputs
+┌─────────────┐              ┌────────▼───┴───────┐
+│             │              │                    │                ┌────────────┐
+│  Rekor Log  ├──────────────►   Rekor Sidekick   │ ───────────────► Pager Duty │
+│             │              │                    │                └────────────┘
+└─────────────┘ Pull entries └───────────────┬─┬─┬┘
+                                             │ │ │                 ┌────────────┐
+                                             │ │ └─────────────────► Stdout     │
+                                             │ │                   └────────────┘
+                                             │ │
+                                             │ │                   ┌────────────┐
+                                             │ └───────────────────► Loki       │
+                                             │                     └────────────┘
+                                             │
+                                             │                     ┌────────────┐
+                                             └─────────────────────► ...        │
+                                                                   └────────────┘
+```
+
+Each configured policy should have metadata that helps make the resulting alert
+be as actionable as possible. Perhaps a name that is programmatically readable
+along with description?
+
+## Event Policy Evaluation
+
+Instead of creating a policy engine, we can an established policy evaluation
+engine: Rego policies and Open Policy Agent! This keeps the complexity of
+implementation low and we can lean on existing documentation for Rego policies
+along with Rekor sidekick specific examples to help folks learn about writing
+policies. This approaches keeps policies flexible as well and keeps our
+coupling to the Rekor log formats fairly low and shifts that burden on to
+policies writers.
+
+## Data structures
+
+We need to define a policy and policy violation:
+
+```go
+type Policy struct {
+    // short machine readable name
+    Name string
+
+    // Background on this policy. Meant for humans. Should help make a 
+    // Policy Violation actionable
+    Description string
+
+    // Where is this policy defined? Could file:// for on disk or maybe
+    // https:// for remote?
+    PolicyURI string
+}
+
+type PolicyViolation struct {
+    // What policy was violated? 
+    Policy Policy
+
+    // URI of the rekor entry that violates the policy 
+    EntryURI string
+}
+```
+
+## Interfaces
+
+We should make a very simply interface for each of the output implementations so that
+contributors can very easily add their own. The following makes sense right now?
+
+```go
+type Output interface {
+    Send(PolicyViolation) error
+}
+```

go.mod

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+module github.com/nsmith5/rekorsidekick
+
+go 1.17