Create ListEntry module readme

Lambdas/Lists/ListEntry/README.md

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+# ListEntry Module
+Deals with managing entries in a list. Note that there is no GET here because that is handled in ListGET see [List readme](https://github.com/ClaytonWWilson/Listify/tree/master/Lambdas/Lists/List)
+
+### ListEntryPOST
+Add a specified quanitity of an item to a list.
+#### Minimum expected request body:
+```
+{
+  "productID": 2,
+  "listID": 4,
+  "quantity": -3,
+  "purchased": false
+}
+```
+Other fields may be populated and are simply ignored.
+
+#### Expected query parameters:
+N/A
+
+#### Inputs:
+  - productID: The productID to add to the list
+  - listID: The listID of the list to be modified
+  - quantity: The number of an item to add (may be negative) to the list
+  - purchased: Currently unused field with future potential for allowing user to check off items they have purchased
+  
+#### Returns:
+null
+  
+  
+### ListEntryDELETE
+Removes the specified product from the list
+
+#### Minimum expected request body:
+```
+{
+  "listID": 6,
+  "productID": 5
+}
+```
+Other fields may be populated and are simply ignored.
+
+#### Expected query parameters:
+N/A
+
+#### Inputs:
+  - listID: The listID of the list to be modified
+  - productID: The productID to remove from the list
+  
+#### Returns:
+null
+