Custom Action for sending Tweets (#784)

* Experimental custom Action for sending Tweets

* Added license headers

* Added README file

* Updated package descriptions

Author: hiranya911

.github/actions/send-tweet/README.md

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+# Send Tweet GitHub Action
+
+This is a minimalistic GitHub Action for posting Firebase release announcements
+to Twitter. Simply specify the Twitter API keys along with the Tweet status to
+be posted.
+
+## Inputs
+
+### `status`
+
+**Required** Text of the Tweet to send.
+
+### `consumer-key`
+
+**Required** Consumer API key from Twitter.
+
+### `consumer-secret`
+
+**Required** Consumer API secret key from Twitter.
+
+### `access-token`
+
+**Required** Twitter application access token.
+
+### `access-token-secret`
+
+**Required** Twitter application access token secret.
+
+## Example usage
+
+```
+- name: Send Tweet
+  uses: firebase/firebase-admin-node/.github/actions/send-tweet
+  with:
+    status: >
+      v1.2.3 of @Firebase Admin Node.js SDK is available.
+      Release notes at https://firebase.google.com.
+    consumer-key: ${{ secrets.TWITTER_CONSUMER_KEY }}
+    consumer-secret: ${{ secrets.TWITTER_CONSUMER_SECRET }}
+    access-token: ${{ secrets.TWITTER_ACCESS_TOKEN }}
+    access-token-secret: ${{ secrets.TWITTER_ACCESS_TOKEN_SECRET }}
+```
+
+## Implementation
+
+This Action uses the `twitter` NPM package to send Tweets.
+
+When making a code change remember to run `npm run pack` to rebuild the
+`dist/index.js` file which is the executable of this Action.

.github/actions/send-tweet/action.yml

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+# Copyright 2020 Google Inc.
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+name: 'Send Tweet Action'
+description: 'Send Tweets from GitHub Actions workflows.'
+inputs:
+  repo:
+    status: Status (Tweet) to be posted
+    required: true
+  consumer-key:
+    description: Consumer API key.
+    required: true
+  consumer-secret:
+    description: Consumer API secret key.
+    required: true
+  access-token:
+    description: Application access token.
+    required: true
+  access-token-secret:
+    description: Application access token secret.
+    required: true
+runs:
+  using: 'node12'
+  main: 'dist/index.js'