Import files with the extensions you specify as text assets in Unity 3D.

TextImporter.cs

/* Original code[1] Copyright (c) 2019 Shane Celis[2]
   Licensed under the MIT License[3]

   [1]: https://gist.github.com/shanecelis/5c38ce90cca057e857f415108f5ac75a
   [2]: https://github.com/shanecelis
   [3]: https://opensource.org/licenses/MIT
*/

using UnityEngine;
using UnityEditor.Experimental.AssetImporters;
using System.IO;

/** Do you want to import your own text files? Sure, we all do!

    Unity natively supports text extensions: txt, html, htm, xml, bytes, json
    csv, yaml, and fnt. These are imported as text assets.

    This TextImporter script imports files with the extensions you specify
    as text assets.
 */
[ScriptedImporter(0 /* Version number. Increment when script is changed. */,
                  new [] { "text", "markdown" } /* Filename extensions */)]
public class TextImporter : ScriptedImporter {
  public override void OnImportAsset(AssetImportContext ctx) {
    var asset = new TextAsset(File.ReadAllText(ctx.assetPath));
    ctx.AddObjectToAsset("Main Object" /* This name won't be shown. The filename
                                          will be shown for the main asset. */,
                         asset);
    ctx.SetMainObject(asset);
  }
}