Add Flow definition file.

src/index.flow.js

@@ -0,0 +1,54 @@
+declare export interface FluxStandardAction<Payload, Meta>{
+  /**
+   * The `type` of an action identifies to the consumer the nature of the action that has occurred.
+   * Two actions with the same `type` MUST be strictly equivalent (using `===`)
+   */
+  type: string,
+
+  /**
+   * The optional `payload` property MAY be any type of value.
+   * It represents the payload of the action.
+   * Any information about the action that is not the type or status of the action should be part of the `payload` field.
+   * By convention, if `error` is `true`, the `payload` SHOULD be an error object.
+   * This is akin to rejecting a promise with an error object.
+  */
+  payload?: Payload,
+
+  /**
+   * The optional `error` property MAY be set to true if the action represents an error.
+   * An action whose `error` is true is analogous to a rejected Promise.
+   * By convention, the `payload` SHOULD be an error object.
+   * If `error` has any other value besides `true`, including `undefined`, the action MUST NOT be interpreted as an error.
+  */
+  error?: boolean,
+
+  /**
+   * The optional `meta` property MAY be any type of value.
+   * It is intended for any extra information that is not part of the payload.
+   */
+  meta?: Meta
+}
+
+declare export type ErrorFluxStandardAction<CustomError, Meta>= {
+  error: true
+} & FluxStandardAction
+
+/**
+ * Alias for FluxStandardAction.
+ */
+declare export type FSA<Payload, Meta>= FluxStandardAction<Payload, Meta>;
+
+/**
+ * Alias for ErrorFluxStandardAction.
+ */
+declare export type ErrorFSA<CustomError, Meta>= ErrorFluxStandardAction<CustomError, Meta>;
+
+/**
+ * Returns `true` if `action` is FSA compliant.
+ */
+declare export function isFSA<Payload, Meta>(action: any): FluxStandardAction
+
+/**
+ * Returns `true` if `action` is FSA compliant error.
+ */
+declare export function isError<CustomError, Meta>(action: any): ErrorFluxStandardAction