add help task

Signed-off-by: Andrey Nering <andrey.nering@gmail.com>

README.md

@@ -268,6 +268,37 @@ printos:
     - echo 'Is SH? {{IsSH}}'
 ```
 
+### Help
+
+Running `task help` lists all tasks with a description. The following taskfile:
+
+```yml
+build:
+  desc: Build the go binary.
+  cmds:
+    - go build -v -i main.go
+
+test:
+  desc: Run all the go tests.
+  cmds:
+    - go test -race ./...
+
+js:
+  cmds:
+    - npm run buildjs
+
+css:
+  cmds:
+    - npm run buildcss
+```
+
+would print the following output:
+
+```bash
+build   Build the go binary.
+test    Run all the go tests.
+```
+
 ## Alternatives
 
 Similar software:

task.go

@@ -4,8 +4,10 @@ import (
 	"fmt"
 	"log"
 	"os"
+	"sort"
 	"strings"
 	"sync"
+	"text/tabwriter"
 
 	"github.com/go-task/task/execext"
 
@@ -30,6 +32,7 @@ var (
 type Task struct {
 	Cmds      []string
 	Deps      []string
+	Desc      string
 	Sources   []string
 	Generates []string
 	Dir       string
@@ -73,6 +76,11 @@ func RunTask(name string) error {
 
 	t, ok := Tasks[name]
 	if !ok {
+		tasks := tasksWithDesc()
+		if len(tasks) > 0 {
+			help(tasks)
+			return nil
+		}
 		return &taskNotFoundError{name}
 	}
 
@@ -202,3 +210,24 @@ func (t *Task) runCommand(i int) error {
 	}
 	return nil
 }
+
+func help(tasks []string) {
+	w := new(tabwriter.Writer)
+	// Format in tab-separated columns with a tab stop of 8.
+	w.Init(os.Stdout, 0, 8, 0, '\t', 0)
+	for _, task := range tasks {
+		fmt.Fprintln(w, fmt.Sprintf("%s\t%s", task, Tasks[task].Desc))
+	}
+	w.Flush()
+}
+
+func tasksWithDesc() []string {
+	tasks := []string{}
+	for name, task := range Tasks {
+		if len(task.Desc) > 0 {
+			tasks = append(tasks, name)
+		}
+	}
+	sort.Strings(tasks)
+	return tasks
+}