Add docs on footnote option

Author: wooorm

index.d.ts

@@ -7,7 +7,7 @@ export {default} from './lib/index.js'
  * Configuration for `remark-gfm`.
  *
  * Currently supports `singleTilde` as a parse option and
- * `tableCellPadding`, `tablePipeAlign`, and `stringLength` as
- * a serialization option.
+ * `firstLineBlank`, `stringLength`, `tableCellPadding`, and `tablePipeAlign`
+ * as serialization options.
  */
 export interface Options extends MicromarkOptions, MdastOptions {}

readme.md

@@ -219,6 +219,8 @@ Configuration (TypeScript type).
 
 ###### Fields
 
+* `firstLineBlank` (`boolean`, default: `false`)
+  — serialize with a blank line for the first line of footnote definitions
 * `stringLength` (`((value: string) => number)`, default: `d => d.length`)
   — detect the size of table cells, used when aligning cells
 * `singleTilde` (`boolean`, default: `true`)