feat: add option for custom release note header

[superewald]

Description

Simple feature to allow adding a custom header for the release notes which is not related to the changelog itself. (As requested in #263).

Usage

You can add a custom header by providing the header option to pluginConfig.
The header option should be either:

• a string (which is inserted at top of release notes)
• a relative file path to a text file (useful for auto-generating the header)

The string or contents of the file will be inserted at the top of the release notes itself.

example config

From string

{ "plugins": [ "@semantic-release/commit-analyzer", ["@semantic-release/release-notes-generator", { "header": "# My awesome custom header" }] ] } 

From file

 "plugins": [ "@semantic-release/commit-analyzer", ["@semantic-release/release-notes-generator", { "header": "./my/custom/header.md" }] ] } 

[EdieLemoine]

Can this be picked up? We'd love to have this feature.