Add theme file for easy install

[cloventt]

With the addition of a theme.toml file, it is now possible to install the add-on as a git submodule in the "standard" way. These are the new installation instructions:

Check out this repo into your themes/ folder:

git submodule add git@github.com:liwenyip/hugo-easy-gallery.git themes/easy-gallery

Then update your ./config.toml to load the theme, for example:

theme = ["hugo-coder", "easy-gallery"]

[kishorbhat]

Reading https://gohugo.io/hugo-modules/theme-components/, it looks like if you have multiple themes that have the same filename for static/ and layouts files, the first theme with a given filename is used. While I anticipate this being a rare occurrence, can you add a note to the README that points to that page to refer to in the event of theme conflicts?

Thanks for the PR!