[gnome-todo] todo-txt: document syntax

[Georges Basile Stavracas Neto <gbsneto src gnome org>]

commit c927e77e36c1f06c9c4a219a982fd04745dd4a59
Author: Rohit Kaushik <kaushikrohit325 gmail com>
Date:   Thu Jun 14 19:47:59 2018 +0530

    todo-txt: document syntax

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+# Syntax For Todo.txt
+
+This document describes the syntax that users should follow when manually 
+editing a Todo.txt managed by GNOME To Do. The rules are based on general
+todo.txt rules. Any deviation from the same is clearly mentioned.
+The deviation from common todo.txt rules is due to GNOME To Do specific
+features and behaviour.
+
+## Line Type
+
+Currently we have general lines with task desctiption and custom
+lines with GNOME To Do specific descriptions. The custom lines
+start with h:1 and are intended not to be edited by the users.
+
+  1) The task line describing a task
+  2) A List line specifying names of all the lists (Starts with `h:1 Lists`)
+  3) A Color lines storing lists color (Starts with `h:1 Colors`)
+
+The format of each task line is specified below
+
+### Task Line
+
+indent x (Priority) Creation-Date Completion-Date title due:date note:"description" @List +parent-task
+
+    :indent               - (Optional) 4 Spaces to indent a task as subtask of previous line.
+                             Don't use tabs for indenting tasks.
+       :x                        - (Optional) Marks the task as completed
+       :Priority                 - (Optional) Should appear in closed parenthesis, can be 
+                                                    A(high), B(medium), C(low) and any other character is 
taken as 
+                                                    default no priority
+                                                    (GNOME To Do Supports 3 priority level High, Medium and 
low)
+       :Creation-Date        - (Optional) Date on which the task was created
+       :Completion-Date      - (Optional) Should be mentioned if Creation Date is mentioned
+       :title                - (Required) Title describing the task
+       :due:date             - (Optional) Due Date of the task
+       :note:"description"   - (Optional) Long Description for the task
+       :@List                - (Required) The Context of the task. Although todo.txt doesn't
+                                                   requires a context to be mentioned but since every task in
+                                                   GNOME To Do belongs to a list and hence it is necessary.
+
+## Custom Lines
+
+Custom Lines appears the end of todo.txt and starts with h:1. At the present To Do specifies
+two custom lines for storing tasklists and their color.
+
+### List Line
+
+The list line starts with "h:1 Lists" and follows with list names. The list name start with
+a "@".
+For example: `h:1 Lists \@Personal \@Work \@Shopping`
+
+The above example contains 3 lists or context namely Personal, Work
+and Shopping
+
+### Color Line
+
+The Color line describes background color for GNOME To Do list. It starts with a
+prefix h:1 Colors and color desciption appears as listname:hexcode and are separated
+by space.
+
+For example: `h:1 Colors Personal:#000000 Work:#ffffff Shopping:#0000ff`
+
+In the above example the color associated with each list or context is as:
+Personal → #000000
+Work     → #ffffff
+Shopping → #0000ff