Add explanation about required parameter
About POST on /menus/:menu_id/items, require fields are marked
About POST on /customers/notes, require fields are marked
About POST on /accounts/:account_id/checkouts, require fields are marked
About POST on /accounts/:account_id/cashiers, required fields are marked
About POST on /accounts/:account_id/customer_tags, required fields are marked
About POST on /accounts/:account_id/payment_types, required fields are marked
About POST on /menus/:menu_id/categories, required fields are marked
About POST on /accounts/:account_id/stocks/events, required fields are marked
Fix mistake
Stop to use :required. Instead, use :optional for optional field
Merge branch 'r/fix-api-document' into gh-pages
Merge pull request #69 from kmizu/gh-pages

POST時に省略可能なパラメータを明記するように修正