Add tempDir option to documentation

[lapwat]

I noticed that there is a tempDir option that you can use to specify a place to put temporary files used by the script.

epub-gen/lib/index.coffee

Line 73 in 1a14d7b

if not @options.tempDir

It would be useful to refer to it in the README.

[lapwat]

A better default value for tempDir would be to use OS specific directory:

https://nodejs.org/api/all.html#fs_fs_mkdtempsync_prefix_options

// require fs, os, path

const prefix = path.join(os.tmpdir(), 'epub-gen-');
const tempDir = fs.mkdtempSync(prefix);

// work with tempDir

fs.rmSync(tempDir, { recursive: true, force: true });