allowUpdates: An optional flag which indicates if we should update a release if it already exists. Defaults to false.
artifact: An optional set of paths representing artifacts to upload to the release. This may be a single path or a comma delimited list of paths (or globs).
artifacts: An optional set of paths representing artifacts to upload to the release. This may be a single path or a comma delimited list of paths (or globs).
artifactContentType: The content type of the artifact. Defaults to raw.
body: An optional body for the release.
bodyFile: An optional body file for the release. This should be the path to the file.
commit: An optional commit reference. This will be used to create the tag if it does not exist.
draft: Optionally marks this release as a draft release. Set to true to enable.
name: An optional name for the release. If this is omitted the tag will be used.
tag: An optional tag for the release. If this is omitted the git ref will be used (if it is a tag).
token: (Required) The Github token. Typically this will be ${{ secrets.GITHUB_TOKEN }}.

Example

This example will create a release when tag is pushed:

name: Releases

on: 
  push:
    tags:
    - '*'

jobs:

  build:
    runs-on: ubuntu-latest
    steps:
    - uses: actions/checkout@v1
    - uses: ncipollo/release-action@v1
      with:
        artifacts: "release.tar.gz,foo/*.txt"
        bodyFile: "body.md"
        token: ${{ secrets.GITHUB_TOKEN }}

Notes

You must provide a tag either via the action input or the git ref (i.e push / create a tag). If you do not the action will fail.
If the tag of the release you are creating does not exist yet, you should set both the tag and commit action inputs. commit can point to a commit hash or a branch name (ex - master).