Added example "Create with custom fields"

[manuelbcd]

Description

New example "examples/createwithcustomfields"

Information that is useful here:

• The What: New example demonstrating how to create issues with custom fields.
• The Why: There were no examples.
• Type of change: Improvement/examples
• Breaking change: No
• Related to an issue: Documentation for custom jira fields #350
• Jira Version + Type: Cloud

Example:

Simply try examples/createwithcustomfields and provide required data (url, user, password, custom field name, custom field value).

go run examples/createwithcustomfields/main.go

[manuelbcd]

Thank you for this nice feedback @benjivesterby. I will update the PR and take the opportunity to submit new PRs applying these changes to the other existing examples

[manuelbcd]

@benjivesterby FYI I have fixed your two first comments. For the third one "_, err := ioutil.ReadAll(resp.Body)" I have restored the error managment to be similar to "create" example, I can evolve it in another PR if you want.

[benjivesterby]

This looks great @manuelbcd thank you for all the hard work!