SessionsPolicy and sessions adaptor, history and _example written.

Former-commit-id: e8b0dde3cb3b72919f01b9d836d8ccb3d4e20214

adaptors/sessions/sessiondb/README.md

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+## Session databases
+
+Find more databases at [github.com/kataras/go-sessions/sessiondb](https://github.com/kataras/go-sessions/tree/master/sessiondb).
+
+This folder contains only the redis database because the rest (two so far, 'file' and 'leveldb') were created by the Community.
+So go [there](https://github.com/kataras/go-sessions/tree/master/sessiondb) and find more about them. `Database` is just an
+interface so you're able to `UseDatabase(anyCompatibleDatabase)`. A Database should implement two functions, `Load` and `Update`.
+
+**Database interface**
+
+```go
+type Database interface {
+	Load(string) map[string]interface{}
+	Update(string, map[string]interface{})
+}
+```
+
+```go
+import (
+  "...myDatabase"
+)
+s := New(...)
+s.UseDatabase(myDatabase) // <---
+
+app := iris.New()
+app.Adapt(s)
+
+app.Listen(":8080")
+```