Mark generated files as such (#75)

Generated files should contain the following comment in the first few
lines for them to be considered generated by GitHub.

    Code generated by <executable>

The additional `@generated` tag does the same for Phabricator.
diff --git a/error.go b/error.go
index fb66bd5..8657849 100644
--- a/error.go
+++ b/error.go
@@ -1,3 +1,5 @@
+// @generated Code generated by gen-valuewrapper.
+
 // Copyright (c) 2020 Uber Technologies, Inc.
 //
 // Permission is hereby granted, free of charge, to any person obtaining a copy
diff --git a/int32.go b/int32.go
index cdd2844..626c833 100644
--- a/int32.go
+++ b/int32.go
@@ -1,3 +1,5 @@
+// @generated Code generated by gen-atomicint.
+
 // Copyright (c) 2020 Uber Technologies, Inc.
 //
 // Permission is hereby granted, free of charge, to any person obtaining a copy
diff --git a/int64.go b/int64.go
index 41c7fef..bfeb055 100644
--- a/int64.go
+++ b/int64.go
@@ -1,3 +1,5 @@
+// @generated Code generated by gen-atomicint.
+
 // Copyright (c) 2020 Uber Technologies, Inc.
 //
 // Permission is hereby granted, free of charge, to any person obtaining a copy
diff --git a/internal/gen-atomicint/main.go b/internal/gen-atomicint/main.go
index 39ca6c8..2ffcab6 100644
--- a/internal/gen-atomicint/main.go
+++ b/internal/gen-atomicint/main.go
@@ -103,7 +103,9 @@
 	return err
 }
 
-var _tmpl = template.Must(template.New("int.go").Parse(`// Copyright (c) 2020 Uber Technologies, Inc.
+var _tmpl = template.Must(template.New("value.go").Parse(`// @generated Code generated by gen-atomicint.
+
+// Copyright (c) 2020 Uber Technologies, Inc.
 //
 // Permission is hereby granted, free of charge, to any person obtaining a copy
 // of this software and associated documentation files (the "Software"), to deal
diff --git a/internal/gen-valuewrapper/main.go b/internal/gen-valuewrapper/main.go
index 24a8f0f..520701e 100644
--- a/internal/gen-valuewrapper/main.go
+++ b/internal/gen-valuewrapper/main.go
@@ -99,7 +99,9 @@
 	return err
 }
 
-var _tmpl = template.Must(template.New("int.go").Parse(`// Copyright (c) 2020 Uber Technologies, Inc.
+var _tmpl = template.Must(template.New("int.go").Parse(`// @generated Code generated by gen-valuewrapper.
+
+// Copyright (c) 2020 Uber Technologies, Inc.
 //
 // Permission is hereby granted, free of charge, to any person obtaining a copy
 // of this software and associated documentation files (the "Software"), to deal
diff --git a/string.go b/string.go
index 1b8c49c..8829c48 100644
--- a/string.go
+++ b/string.go
@@ -1,3 +1,5 @@
+// @generated Code generated by gen-valuewrapper.
+
 // Copyright (c) 2020 Uber Technologies, Inc.
 //
 // Permission is hereby granted, free of charge, to any person obtaining a copy
diff --git a/uint32.go b/uint32.go
index aa5d5b1..610d9e7 100644
--- a/uint32.go
+++ b/uint32.go
@@ -1,3 +1,5 @@
+// @generated Code generated by gen-atomicint.
+
 // Copyright (c) 2020 Uber Technologies, Inc.
 //
 // Permission is hereby granted, free of charge, to any person obtaining a copy
diff --git a/uint64.go b/uint64.go
index 3771d96..3ebf3c7 100644
--- a/uint64.go
+++ b/uint64.go
@@ -1,3 +1,5 @@
+// @generated Code generated by gen-atomicint.
+
 // Copyright (c) 2020 Uber Technologies, Inc.
 //
 // Permission is hereby granted, free of charge, to any person obtaining a copy