Update Contributing Guidelines

### Problem

When creating new Pull requests I've noticed that people are guided to branch off the `main` branch.
"
4. Sync your local `main` branch.

```sh
$ git pull upstream main
```

> **_Note_** Some repositories may still be using `master` for their default branch.

5. Create a branch to add a new feature or fix issues.

```sh
$ git checkout -b new-feature
```
"

This is not really precise. As I can see everyone starts from develop and is creating PR to develop as well (https://github.com/hashgraph/hedera-services/pulls?q=is%3Apr+is%3Aclosed).

### Solution

Create new file `CONTRIBUTING.md` inside the hedera-services repository, where starting branch will be stated explicitly.

### Alternatives

...or we can ignore it, but this intruduces unneccessary confusion among new contrubutors