Add docs for no-access-state-in-setstate

Author: jaaberg

README.md

@@ -88,6 +88,7 @@ Finally, enable all of the rules that you would like to use.  Use [our preset](#
 * [react/forbid-elements](docs/rules/forbid-elements.md): Forbid certain elements
 * [react/forbid-prop-types](docs/rules/forbid-prop-types.md): Forbid certain propTypes
 * [react/forbid-foreign-prop-types](docs/rules/forbid-foreign-prop-types.md): Forbid foreign propTypes
+* [react/no-access-state-in-setstate](docs/rules/no-access-state-in-setstate.md): Prevent using this.state inside this.setState
 * [react/no-array-index-key](docs/rules/no-array-index-key.md): Prevent using Array index in `key` props
 * [react/no-children-prop](docs/rules/no-children-prop.md): Prevent passing children as props
 * [react/no-danger](docs/rules/no-danger.md): Prevent usage of dangerous JSX properties

docs/rules/no-access-state-in-setstate.md

@@ -0,0 +1,39 @@
+# Prevent using this.state within a this.setState (no-access-state-in-setstate)
+
+This rule should prevent usage of this.state inside setState calls.
+Such usage of this.state might result in errors when two state calls is
+called in batch and thus referencing old state and not the current
+state. An example can be an increment function:
+
+```
+function increment() {
+  this.setState({value: this.state.value + 1});
+}
+```
+
+If these two setState operations is grouped together in a batch it will
+look be something like the following, given that value is 1:
+
+```
+setState({value: 1 + 1})
+setState({value: 1 + 1})
+```
+
+This can be avoided with using callbacks which takes the previous state
+as first argument:
+
+```
+function increment() {
+  this.setState(prevState => ({value: prevState.value + 1}));
+}
+```
+
+Then react will call the argument with the correct and updated  state,
+even when things happen in batches. And the example above will be
+something like:
+
+
+```
+setState({value: 1 + 1})
+setState({value: 2 + 1})
+```