Added README file

Author: randilfernando

README.md

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+# Gitter notify - Github action
+
+A [GitHub Action](https://github.com/features/actions) to send a message to a Gitter room.
+
+**Example**
+
+<img width="485" alt="gitter-github-action-example" src="https://user-images.githubusercontent.com/17607423/79047650-97327a00-7c35-11ea-8ebb-5348ea23fbc8.png">
+
+
+## Usage
+
+You can use this action after any other action. Here is an example setup of this action:
+
+1. Create a `.github/workflows/gitter-notify.yml` file in your GitHub repo.
+2. Add the following code to the `gitter-notify.yml` file.
+
+```yml
+on: [push]
+
+jobs:
+  send_message:
+    runs-on: ubuntu-latest
+    name: Send test message
+
+    steps:
+    - id: initial
+      uses: juztcode/gitter-github-action@v1
+      with:
+        room-id: ${{ secrets.GITTER_ROOM_ID }}
+        token: ${{ secrets.GITTER_TOKEN }}
+        text: Pending
+
+    - uses: juztcode/gitter-github-action@v1
+      with:
+        room-id: ${{ secrets.GITTER_ROOM_ID }}
+        token: ${{ secrets.GITTER_TOKEN }}
+        text: Success
+        message-id: ${{ steps.initial.outputs.message-id }}
+```
+
+3. Create `GITTER_ROOM_ID` and `GITTER_TOKEN` secrets using [GitHub Action's Secret](https://developer.github.com/actions/creating-workflows/storing-secrets). 
+4. You can get Gitter personal access token from [Gitter Developer Page](https://developer.gitter.im/apps).
+
+
+## Inputs
+
+Input             | Purpose
+------------------|---------------------------------------------------------------------------------------------------------------------------------------
+room-id           | Specify Gitter room id in which message needs to be sent
+token             | Personal access token of a user who has permission to send messages to above mentioned Gitter room.
+text              | Message need to be displayed (Ref, EventName, ActionUrl are automatically added to the message).
+message-id        | Instead of creating a new message if you want to replace existing then you can provide id. After github action submitted message id can be accessed via action outputs.
+
+>**At the moment Gitter developer api does not provide option to reset personal access token**