Added example for using context in class component (piotrwitek#141)

Fixes piotrwitek#140 

Let me know if it looks good or I need to make any changes. Thanks!

Author: binoy14

README.md

@@ -845,6 +845,30 @@ export default function ToggleThemeButton(props: Props) {
 
 ```
 
+#### ThemeConsumer in class component
+
+```tsx
+import * as React from 'react';
+import ThemeContext from './theme-context';
+
+type Props = {};
+
+export class ToggleThemeButtonClass extends React.Component<Props> {
+  static contextType = ThemeContext;
+  context!: React.ContextType<typeof ThemeContext>;
+
+  render() {
+    const { theme, toggleTheme } = this.context;
+    return (
+      <button style={theme} onClick={toggleTheme}>
+        Toggle Theme
+      </button>
+    );
+  }
+}
+
+```
+
 [Implementation with Hooks](#--usecontext)
 
 [⇧ back to top](#table-of-contents)

README_SOURCE.md

@@ -319,6 +319,10 @@ const mapDispatchToProps = (dispatch: Dispatch<ActionType>) => ({
 
 ::codeblock='playground/src/context/theme-consumer.tsx'::
 
+#### ThemeConsumer in class component
+
+::codeblock='playground/src/context/theme-consumer-class.tsx'::
+
 [Implementation with Hooks](#--usecontext)
 
 [⇧ back to top](#table-of-contents)

playground/src/context/theme-consumer-class.tsx

@@ -0,0 +1,18 @@
+import * as React from 'react';
+import ThemeContext from './theme-context';
+
+type Props = {};
+
+export class ToggleThemeButtonClass extends React.Component<Props> {
+  static contextType = ThemeContext;
+  context!: React.ContextType<typeof ThemeContext>;
+
+  render() {
+    const { theme, toggleTheme } = this.context;
+    return (
+      <button style={theme} onClick={toggleTheme}>
+        Toggle Theme
+      </button>
+    );
+  }
+}