When your GitHub projects have no License file on it and you tried to submit your projects as pods to CocoaPods.
In the pod sepc lint /your project/ process, you will get a warning like this:

WARN | [iOS] Unable to find a license file

The first thing you have to do is to push a License file on GitHub. For example if your open source license goes for MIT License it will looks like this:

Version 1.0

Created by Saiday on 01/14/2013.
Copyright 2013 Saiday

This code is distributed under the terms and conditions of the MIT license.

Permission is hereby granted, free of charge, to any person obtaining a copy
of this software and associated documentation files (the "Software"), to deal
in the Software without restriction, including without limitation the rights
to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
copies of the Software, and to permit persons to whom the Software is
furnished to do so, subject to the following conditions:

The above copyright notice and this permission notice shall be included in
all copies or substantial portions of the Software.

THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
THE SOFTWARE.
GitHub Tag
Notice that if you already tag your projects for CocoaPods pod version, you have to re-tag it after you pushed the License file.

Try lint again
Try pod spec lint /your project/ again, if you still get the WARN | [iOS] Unable to find a license file warning, it because of CocoaPods caches issue.

Delete CocoaPods caches data
rm -rf /Users/YourName/Library/Caches/CocoaPods/GitHub/

then

mkdir /Users/YourName/Library/Caches/CocoaPods/GitHub/

Now, it should works.