Add license information to the website

[johannesjh]

As a new user of Planner, I found it difficult to find out license information. I was then pleasantly surprised when I found out that this is an open source project. Hence my suggestion is to make it more clear on the website that Planner is open source.

Specific clues that I was looking for (but did not find on the website) include:

• the word "open source"
• the word "license"
• a link to the github repository
• an invitation that encourages contributions