✅ Add an option to run tests in watch mode

Also add some explanations on how to run tests for a specific exercise only.

README.md

@@ -11,5 +11,16 @@ To run the tests, simply run
 $ yarn test
 ```
 
+You can also run them in watch mode:
+```sh
+$ yarn test:watch
+```
+
+Finally, if you wish to only run the tests for a given exercice `exoN`, you can
+run the following:
+```sh
+$ yarn test[:watch] exoN
+```
+
 The exercices are organized into `exoN` folders and most of what is required to
 complete each is detailed in the comments.

package.json

@@ -12,7 +12,8 @@
   "homepage": "https://github.com/inato/fp-ts-training",
   "scripts": {
     "build": "yarn tsc -b",
-    "test": "jest --runInBand"
+    "test": "jest --runInBand",
+    "test:watch": "jest --runInBand --watch"
   },
   "dependencies": {
     "decod": "^6.1.6",