Adding migration guide

Author: nicksrandall

MIGRATE.md

@@ -0,0 +1,88 @@
+## Upgrade from v1 to v2
+The only difference between v1 and v2 is that we added use of [context](https://golang.org/pkg/context).
+
+```diff
+- loader.Load(key string) Thunk
++ loader.Load(ctx context.Context, key string) Thunk
+- loader.LoadMany(keys []string) ThunkMany
++ loader.LoadMany(ctx context.Context, keys []string) ThunkMany
+```
+
+```diff
+- type BatchFunc func([]string) []*Result
++ type BatchFunc func(context.Context, []string) []*Result
+```
+
+## Upgrade from v2 to v3
+```diff
+// dataloader.Interface as added context.Context to methods
+- loader.Prime(key string, value interface{}) Interface
++ loader.Prime(ctx context.Context, key string, value interface{}) Interface
+- loader.Clear(key string) Interface
++ loader.Clear(ctx context.Context, key string) Interface
+```
+
+```diff
+// cache interface as added context.Context to methods
+type Cache interface {
+-	Get(string) (Thunk, bool)
++	Get(context.Context, string) (Thunk, bool)
+-	Set(string, Thunk)
++	Set(context.Context, string, Thunk)
+-	Delete(string) bool
++	Delete(context.Context, string) bool
+	Clear()
+}
+```
+
+## Upgrade from v3 to v4
+```diff
+// dataloader.Interface as now allows interace{} as key rather than string
+- loader.Load(context.Context, key string) Thunk
++ loader.Load(ctx context.Context, key interface{}) Thunk
+- loader.LoadMany(context.Context, key []string) ThunkMany
++ loader.LoadMany(ctx context.Context, keys []interface{}) ThunkMany
+- loader.Prime(context.Context, key string, value interface{}) Interface
++ loader.Prime(ctx context.Context, key interface{}, value interface{}) Interface
+- loader.Clear(context.Context, key string) Interface
++ loader.Clear(ctx context.Context, key interface{}) Interface
+```
+
+```diff
+// cache interface now allows interface{} as key instead of string
+type Cache interface {
+-	Get(context.Context, string) (Thunk, bool)
++	Get(context.Context, interface{}) (Thunk, bool)
+-	Set(context.Context, string, Thunk)
++	Set(context.Context, interface{}, Thunk)
+-	Delete(context.Context, string) bool
++	Delete(context.Context, interface{}) bool
+	Clear()
+}
+```
+
+## Upgrade from v4 to v5
+```diff
+// dataloader.Interface as now allows interace{} as key rather than string
+- loader.Load(context.Context, key interface{}) Thunk
++ loader.Load(ctx context.Context, key Key) Thunk
+- loader.LoadMany(context.Context, key []interface{}) ThunkMany
++ loader.LoadMany(ctx context.Context, keys Keys) ThunkMany
+- loader.Prime(context.Context, key interface{}, value interface{}) Interface
++ loader.Prime(ctx context.Context, key Key, value interface{}) Interface
+- loader.Clear(context.Context, key interface{}) Interface
++ loader.Clear(ctx context.Context, key Key) Interface
+```
+
+```diff
+// cache interface now allows interface{} as key instead of string
+type Cache interface {
+-	Get(context.Context, interface{}) (Thunk, bool)
++	Get(context.Context, Key) (Thunk, bool)
+-	Set(context.Context, interface{}, Thunk)
++	Set(context.Context, Key, Thunk)
+-	Delete(context.Context, interface{}) bool
++	Delete(context.Context, Key) bool
+	Clear()
+}
+```