Adjust Tag syntax to match TaskWarrior

docs/src/SUMMARY.md

@@ -5,6 +5,7 @@
   * [Using the Task Command](./using-task-command.md)
     * [Configuration](./config-file.md)
     * [Reports](./reports.md)
+    * [Tags](./tags.md)
     * [Synchronization](./task-sync.md)
   * [Running the Sync Server](./running-sync-server.md)
   * [Debugging](./debugging.md)

docs/src/tags.md

@@ -0,0 +1,12 @@
+# Tags
+
+Each task has a collection of associated tags.
+Tags are short words that categorize tasks, typically written with a leading `+`, such as `+next` or `+jobsearch`.
+
+Tags are useful for filtering tasks in reports or on the command line.
+For example, when it's time to continue the job search, `task +jobsearch` will show pending tasks with the `jobsearch` tag.
+
+## Allowed Tags
+
+Specifically, tags must be at least one character long and cannot contain whitespace or any of the characters `+-*/(<>^! %=~`.
+The first character cannot be a digit, and `:` is not allowed after the first character.