Add warning on file prop object equality check

Closes wojtekmaj#308

README.md

@@ -206,7 +206,7 @@ Loads a document passed using `file` prop.
 |className|Class name(s) that will be added to rendered element along with the default `react-pdf__Document`.|n/a|<ul><li>String:<br />`"custom-class-name-1 custom-class-name-2"`</li><li>Array of strings:<br />`["custom-class-name-1", "custom-class-name-2"]`</li></ul>|
 |error|What the component should display in case of an error.|`"Failed to load PDF file."`|<ul><li>String:<br />`"An error occurred!"`</li><li>React element:<br />`<div>An error occurred!</div>`</li><li>Function:<br />`this.renderError`</li></ul>|
 |externalLinkTarget|Link target for external links rendered in annotations.|unset, which means that default behavior will be used|One of valid [values for `target` attribute](https://developer.mozilla.org/en-US/docs/Web/HTML/Element/a#Attributes).<ul><li>`"_self"`</li><li>`"_blank"`</li><li>`"_parent"`</li><li>`"_top"`</li></ul>
-|file|What PDF should be displayed.<br />Its value can be an URL, a file (imported using `import ... from ...` or from file input form element), or an object with parameters (`url` - URL; `data` - data, preferably Uint8Array; `range` - PDFDataRangeTransport; `httpHeaders` - custom request headers, e.g. for authorization), `withCredentials` - a boolean to indicate whether or not to include cookies in the request (defaults to `false`).|n/a|<ul><li>URL:<br />`"http://example.com/sample.pdf"`</li><li>File:<br />`import sample from '../static/sample.pdf'` and then<br />`sample`</li><li>Parameter object:<br />`{ url: 'http://example.com/sample.pdf', httpHeaders: { 'X-CustomHeader': '40359820958024350238508234' }, withCredentials: true }`</ul>|
+|file|What PDF should be displayed.<br />Its value can be an URL, a file (imported using `import ... from ...` or from file input form element), or an object with parameters (`url` - URL; `data` - data, preferably Uint8Array; `range` - PDFDataRangeTransport; `httpHeaders` - custom request headers, e.g. for authorization), `withCredentials` - a boolean to indicate whether or not to include cookies in the request (defaults to `false`).<br />**Warning**: Since equality check (`===`) is used to determine if `file` object has changed, it must be memoized by setting it in component's state, `useMemo` or other similar technique.|n/a|<ul><li>URL:<br />`"http://example.com/sample.pdf"`</li><li>File:<br />`import sample from '../static/sample.pdf'` and then<br />`sample`</li><li>Parameter object:<br />`{ url: 'http://example.com/sample.pdf', httpHeaders: { 'X-CustomHeader': '40359820958024350238508234' }, withCredentials: true }`</ul>|
 |inputRef|A function that behaves like ref, but it's passed to main `<div>` rendered by `<Document>` component.|n/a|`(ref) => { this.myDocument = ref; }`|
 |loading|What the component should display while loading.|`"Loading PDF…"`|<ul><li>String:<br />`"Please wait!"`</li><li>React element:<br />`<div>Please wait!</div>`</li><li>Function:<br />`this.renderLoader`</li></ul>|
 |noData|What the component should display in case of no data.|`"No PDF file specified."`|<ul><li>String:<br />`"Please select a file."`</li><li>React element:<br />`<div>Please select a file.</div>`</li><li>Function:<br />`this.renderNoData`</li></ul>|