add readme contents

README.md

@@ -7,6 +7,39 @@ This includes tools for developing implementations of the parser interface, for
 ## Implementations
 
 * [JSON](https://github.com/katydid/parser-go-json)
+* [Protobufs](https://github.com/katydid/parser-go-proto)
 * [XML](https://github.com/katydid/parser-go-xml)
-* [Reflect](https://github.com/katydid/parser-go-reflect)
 * [YAML](https://github.com/katydid/parser-go-yaml)
+* [Reflect](https://github.com/katydid/parser-go-reflect)
+
+## Implementing your own parser
+
+The katydid validator supports validating any serialization format that implements the following parser interface:
+
+```go
+type Interface interface {
+    Next() error
+    IsLeaf() bool
+    Up()
+    Down()
+    Value
+}
+
+type Value interface {
+    Double() (float64, error)
+    Int() (int64, error)
+    Uint() (uint64, error)
+    Bool() (bool, error)
+    String() (string, error)
+    Bytes() ([]byte, error)
+}
+```
+
+This interface allows for the implementation of an online pull based parser. 
+That is a parser that lazily parses the input as the methods are called and only parses the input once, without backtracking. 
+Exercising the parser can be done with the [debug.Walk](https://pkg.go.dev/github.com/katydid/parser-go/parser/debug#Walk) function. 
+The Walk function also returns some debugging output, which should be useful in the development of your own parser.
+
+Your parser should also be able to handle skipping of some of the input. 
+This happens when the Walk function returns before encountering an `io.EOF`. 
+The [debug.RandomWalk](https://pkg.go.dev/github.com/katydid/parser-go/parser/debug#RandomWalk) function is useful for testing this type of robustness in your parser.