Update performance.md (#65)

Call out table capacity prediction

docs/_pages/performance.md

@@ -102,6 +102,16 @@ Iterating through array-like tables using `for i=1,#t` tends to be slightly slow
 
 Luau implements several optimizations for table creation. When creating object-like tables, it's recommended to use table literals (`{ ... }`) and to specify all table fields in the literal in one go instead of assigning fields later; this triggers an optimization inspired by LuaJIT's "table templates" and results in higher performance when creating objects. When creating array-like tables, if the maximum size of the table is known up front, it's recommended to use `table.create` function which can create an empty table with preallocated storage, and optionally fill it with a given value.
 
+When the exact table shape isn't known, Luau compiler can still predict the table capacity required in case the table is initialized with an empty literal (`{}`) and filled with fields subsequently. For example, the following code creates a correctly sized table implicitly:
+
+```lua
+local v = {}
+v.x = 1
+v.y = 2
+v.z = 3
+return v
+```
+
 When appending elements to tables, it's recommended to use `table.insert` (which is the fastest method to append an element to a table if the table size is not known). In cases when a table is filled sequentially, however, it's much more efficient to use a known index for insertion - together with preallocating tables using `table.create` this can result in much faster code, for example this is the fastest way to build a table of squares:
 
 ```lua