docs(gatsby-adapter-netlify): add CHANGELOG.md

[pieh]

Description

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Our automated changelog generator/updater wasn't generating changelog for gatsby-adapter-netlify because there wasn't CHANGELOG.md boilerplate added at the time package was initially introduced - this adds generated version (locally) that should be updated with bot in the future

FRA-265

[pieh]

Lack of above marker (and file in general) was causing changelog generation scripts to skip generating changelog for adapter:

gatsby/scripts/gatsby-changelog-generator/generate.js

Lines 248 to 255 in 222158e

	const path = changelogPath(packageName)
	const separator = `<a name="before-release-process"></a>`
	const contents = String(fs.readFileSync(path))
	const parts = contents.split(separator)
	if (parts.length !== 2) {
	throw new Error(`Could not find demarcation ${separator.trim()} in ${path}`)
	}

gatsby/scripts/gatsby-changelog-generator/generate.js

Line 291 in 222158e

const contents = String(fs.readFileSync(path))

[MarcL]

@pieh - Want to rebase with master and we can get it approved and merged?