Update README.md to include 'valueComponent' prop (JedWatson#1803)

* Update README.md to include 'valueComponent' prop

Thanks to some very helpful feedback I received in this repo (and reading through some issues other users were having) I found there was a valueComponent prop that I could utilize that made my life so much easier.  I think it needs to be included in the list of available props in the docs.

* Update README.md to include 'valueComponent' prop

(edited with suggested changes made)

* Minor copy edits of valueComponent description

Author: lstuartfry

README.md

@@ -400,6 +400,7 @@ function onInputKeyDown(event) {
 | loadingPlaceholder | string\|node | 'Loading...' | label to prompt for loading search result |
 | tabSelectsValue | bool | true | whether to select the currently focused value when the `[tab]` key is pressed |
 | value | any | undefined | initial field value |
+| valueComponent | func | <Value /> | function which returns a custom way to render/manage the value selected `<CustomValue />` |
 | valueKey | string | 'value' | the option property to use for the value |
 | valueRenderer | func | undefined | function which returns a custom way to render the value selected `function (option) {}` |