Add CONTRIBUTING.md

- This makes it easy for someone new to start working on BotKube
- Adds instructions to build BotKube from source code
- Based on
  https://github.com/jaegertracing/jaeger-client-python/blob/master/CONTRIBUTING.md
  which is Apache 2.0 licensed

Signed-off-by: Bhavin Gandhi <bhavin7392@gmail.com>

CONTRIBUTING.md

@@ -0,0 +1,124 @@
+# How to Contribute to BotKube
+
+We'd love your help!
+
+BotKube is [MIT Licensed](LICENSE) and accepts contributions via
+GitHub pull requests. This document outlines some of the conventions
+on development workflow, commit message formatting, contact points and
+other resources to make it easier to get your contributions accepted.
+
+We gratefully welcome improvements to
+[documentation](https://www.botkube.io/ "Go to documentation site") as
+well as to code.
+
+## Contributing to documentation
+You can contribute to documentation by following [these
+instructions](https://github.com/infracloudio/botkube-docs#contributing
+"Contributing to BotKube Docs")
+
+## Compile BotKube from source code
+### Prerequisite
+* Make sure you have `go` compiler installed.
+* BotKube uses [`dep`](https://github.com/golang/dep) to manage
+dependencies.
+* You will also need `make` and
+[`docker`](https://docs.docker.com/install/) installed on your
+machine.
+* Clone the source code
+   ```sh
+   $ git clone https://github.com/infracloudio/botkube.git $GOPATH/src/github.com/infracloudio/botkube
+   $ cd $GOPATH/src/github.com/infracloudio/botkube
+   ```
+
+Now you can build and run BotKube by one of the following ways
+### Build the container image
+1. This will build BotKube and create a new container image tagged as `infracloud/botkube:latest`
+   ```sh
+   $ make build
+   $ docker tag infracloud/botkube:latest <your_account>/botkube:latest
+   $ docker push <your_account>/botkube:latest
+   ```
+   Where `<your_account>` is Docker hub account to which you can push the image
+
+2. Follow the [instructions from
+   README.md](https://github.com/infracloudio/botkube#using-helm) to
+   deploy newly created image in your cluster.
+   ```sh
+   # Set the container image
+   $ helm install --name botkube --namespace botkube \
+	   ... other options ...
+	   --set image.repository=your_account/botkube \
+	   --set image.tag=latest
+	   ...
+	   helm/botkube
+   ```
+
+### Build and run BotKube locally
+1. Build BotKube binary  
+   If you don't want to build the container image, you can build the
+   binary like this,
+   ```sh
+   # Fetch the dependencies
+   $ dep ensure
+   # Build the binary
+   $ go build ./cmd/botkube/
+   ```
+2. Modify `config.yaml` according to your needs. Please refer
+   [configuration section](https://www.botkube.io/configuration/) from
+   documentation for more details
+   ```sh
+   # From project root directory
+   $ xdg-open config.yaml
+   ```
+3. Export the path to directory of `config.yaml`
+   ```sh
+   # From project root directory
+   $ export CONFIG_PATH=$(pwd)
+   ```
+4. Make sure that correct context is set and you are able to access
+   your Kubernetes cluster
+   ```console
+   $ kubectl config current-context
+   minikube
+   $ kubectl cluster-info
+   Kubernetes master is running at https://192.168.39.233:8443
+   CoreDNS is running at https://192.168.39.233:8443/api/v1/namespaces/kube-system/services/kube-dns:dns/proxy
+   ...
+	```
+5. Run BotKube binary
+   ```sh
+   $ ./botkube
+   ```
+
+## Making A Change
+
+* *Before making any significant changes, please [open an
+issue](https://github.com/infracloudio/botkube/issues).* Discussing
+your proposed changes ahead of time will make the contribution process
+smooth for everyone.
+
+* Once we've discussed your changes and you've got your code ready,
+make sure that build steps mentioned above pass. Open your pull
+request against
+[`develop`](http://github.com/infracloudio/botkube/tree/develop)
+branch.
+
+* To avoid build failures in CI, run
+  ```sh
+  # From project root directory
+  $ ./hack/verify-*.sh
+  ```
+  This will check if the code is properly formatted, linted & vendor directory is present.
+
+* Make sure your pull request has [good commit
+messages](https://chris.beams.io/posts/git-commit/):
+  * Separate subject from body with a blank line
+  * Limit the subject line to 50 characters
+  * Capitalize the subject line
+  * Do not end the subject line with a period
+  * Use the imperative mood in the subject line
+  * Wrap the body at 72 characters
+  * Use the body to explain _what_ and _why_ instead of _how_
+
+* Try to squash unimportant commits and rebase your changes on to
+develop branch, this will make sure we have clean log of changes.