docs: add update instructions for 1.20

Signed-off-by: Mark Sagi-Kazar <mark.sagikazar@gmail.com>

UPDATES.md

@@ -0,0 +1,126 @@
+# Update Log
+
+**This document details any major updates required to use new features or improvements in Viper.**
+
+## v1.20.x
+
+### New file searching API
+
+Viper now includes a new file searching API that allows users to customize how Viper looks for config files.
+
+Viper accepts a custom [`Finder`](https://pkg.go.dev/github.com/spf13/viper#Finder) interface implementation:
+
+```go
+// Finder looks for files and directories in an [afero.Fs] filesystem.
+type Finder interface {
+	Find(fsys afero.Fs) ([]string, error)
+}
+```
+
+It is supposed to return a list of paths to config files.
+
+The default implementation uses [github.com/sagikazarmark/locafero](https://github.com/sagikazarmark/locafero) under the hood.
+
+You can supply your own implementation using `WithFinder`:
+
+```go
+v := viper.NewWithOptions(
+    viper.WithFinder(&MyFinder{}),
+)
+```
+
+For more information, check out the [Finder examples](https://pkg.go.dev/github.com/spf13/viper#Finder)
+and the [documentation](https://pkg.go.dev/github.com/sagikazarmark/locafero) for the locafero package.
+
+### New encoding API
+
+Viper now allows customizing the encoding layer by providing an API for encoding and decoding configuration data:
+
+```go
+// Encoder encodes Viper's internal data structures into a byte representation.
+// It's primarily used for encoding a map[string]any into a file format.
+type Encoder interface {
+	Encode(v map[string]any) ([]byte, error)
+}
+
+// Decoder decodes the contents of a byte slice into Viper's internal data structures.
+// It's primarily used for decoding contents of a file into a map[string]any.
+type Decoder interface {
+	Decode(b []byte, v map[string]any) error
+}
+
+// Codec combines [Encoder] and [Decoder] interfaces.
+type Codec interface {
+	Encoder
+	Decoder
+}
+```
+
+By default, Viper includes the following codecs:
+
+- JSON
+- TOML
+- YAML
+- Dotenv
+
+The rest of the codecs are moved to [github.com/go-viper/encoding](https://github.com/go-viper/encoding)
+
+Customizing the encoding layer is possible by providing a custom registry of codecs:
+
+- [Encoder](https://pkg.go.dev/github.com/spf13/viper#Encoder) -> [EncoderRegistry](https://pkg.go.dev/github.com/spf13/viper#EncoderRegistry)
+- [Decoder](https://pkg.go.dev/github.com/spf13/viper#Decoder) -> [DecoderRegistry](https://pkg.go.dev/github.com/spf13/viper#DecoderRegistry)
+- [Codec](https://pkg.go.dev/github.com/spf13/viper#Codec) -> [CodecRegistry](https://pkg.go.dev/github.com/spf13/viper#CodecRegistry)
+
+You can supply the registry of codecs to Viper using the appropriate `With*Registry` function:
+
+```go
+codecRegistry := viper.NewCodecRegistry()
+
+codecRegistry.RegisterCodec("myformat", &MyCodec{})
+
+v := viper.NewWithOptions(
+    viper.WithCodecRegistry(codecRegistry),
+)
+```
+
+### BREAKING: HCL, Java properties, INI removed from core
+
+In order to reduce third-party dependencies, Viper dropped support for the following formats from the core:
+
+- HCL
+- Java properties
+- INI
+
+You can still use these formats though by importing them from [github.com/go-viper/encoding](https://github.com/go-viper/encoding):
+
+```go
+import (
+    "github.com/go-viper/encoding/hcl"
+    "github.com/go-viper/encoding/javaproperties"
+    "github.com/go-viper/encoding/ini"
+)
+
+codecRegistry := viper.NewCodecRegistry()
+
+{
+    codec := hcl.Codec{}
+
+    codecRegistry.RegisterCodec("hcl", codec)
+    codecRegistry.RegisterCodec("tfvars", codec)
+
+}
+
+{
+    codec := &javaproperties.Codec{}
+
+    codecRegistry.RegisterCodec("properties", codec)
+    codecRegistry.RegisterCodec("props", codec)
+    codecRegistry.RegisterCodec("prop", codec)
+}
+
+codecRegistry.RegisterCodec("ini", ini.Codec{})
+
+v := viper.NewWithOptions(
+    viper.WithCodecRegistry(codecRegistry),
+)
+```