Update CONTRIBUTING.md

[varghesejose2020]

added markdown styles to contributing guidelines
Hacktoberfest

[arushi09-hub]

Hi There,
We thank you for your valuable contributions. Please follow the steps below for the Giveaways Entry -

1. Once the PR is raised you can join the discord community for the upcoming updates.
2. After joining the community, send your PR link on the #contrib channel with the tag @hacktoberfest.
3. We will evaluate the PR and once it is accepted , we will send you the swags.