doc: add schema annotation to entgo.io (#880)

doc/md/schema-annotations.md

@@ -0,0 +1,45 @@
+---
+id: schema-annotations
+title: Annotations
+---
+
+Schema annotations allow attaching metadata to schema objects like fields and edges and inject them to external templates.
+An annotation must be a Go type that is serializable to JSON raw value (e.g. struct, map or slice)
+and implement the [Annotation](https://pkg.go.dev/github.com/facebook/ent/schema/field?tab=doc#Annotation) interface.
+
+The builtin annotations allow configuring the different storage drivers (like SQL) and control the code generation output. 
+
+## Custom Table Name
+
+A custom table name can be provided for types using the `entsql` annotation as follows:
+
+```go
+package schema
+
+import (
+	"github.com/facebook/ent"
+	"github.com/facebook/ent/dialect/entsql"
+	"github.com/facebook/ent/schema"
+	"github.com/facebook/ent/schema/field"
+)
+
+// User holds the schema definition for the User entity.
+type User struct {
+	ent.Schema
+}
+
+// Annotations of the User.
+func (User) Annotations() []schema.Annotation {
+	return []schema.Annotation{
+		entsql.Annotation{Table: "Users"},
+	}
+}
+
+// Fields of the User.
+func (User) Fields() []ent.Field {
+	return []ent.Field{
+		field.Int("age"),
+		field.String("name"),
+	}
+}
+```