docs: add %checks and T is x

README.md

@@ -26,7 +26,7 @@ Some of these differences are subjective (e.g. error readability), and I'd love
 | specifying generic parameters during call-time (`f<T>(x)`) | yes [e.g.](http://www.typescriptlang.org/play/#src=function%20someFactory%3CT%3E()%20%7B%0D%0A%20%20return%20class%20%7B%0D%0A%20%20%20%20method(someParam%20%3A%20T)%20%7B%0D%0A%20%20%20%20%20%20%0D%0A%20%20%20%20%7D%0D%0A%20%20%7D%0D%0A%7D%0D%0A%0D%0A%2F%2F%20how%20to%20invoke%20this%20factory%20with%20a%20defined%20%3CT%3E%3F%0D%0A%0D%0Aconst%20SomeClass%20%3D%20someFactory%3C%7B%20whatever%3A%20string%20%7D%3E()%0D%0Aconst%20someInstance%20%3D%20new%20SomeClass()%0D%0AsomeInstance.method('will-error-here')%0D%0A) | yes (since Flow 0.72) |
 | specifying generic parameters for type definitions | yes | yes |
 | typings for public libraries | plenty of well maintained typings | a handful of mostly incomplete typings |
-| unique features | <ul><li>autocomplete for object construction</li><li>declarable `this` in functions (typing `someFunction.bind()`)</li><li>large library of typings</li><li>more flexible [type mapping via iteration](https://github.com/Microsoft/TypeScript/pull/12114)</li><li>namespacing</li></ul> | <ul><li>variance</li><li>existential types `*` (deprecated since 0.72)</li><li>testing potential code-paths when types not declared for maximum inference</li><li>`$Diff<A, B>` type</li></ul> |
+| unique features | <ul><li>autocomplete for object construction</li><li>declarable `this` in functions (typing `someFunction.bind()`)</li><li>large library of typings</li><li>more flexible [type mapping via iteration](https://github.com/Microsoft/TypeScript/pull/12114)</li><li>namespacing</li></ul> | <ul><li>variance</li><li>existential types `*` (deprecated since 0.72)</li><li>testing potential code-paths when types not declared for maximum inference</li><li>`$Diff<A, B>` type</li><li>Predicate function `%checks`, which will instruct the compiler on the non-nullable state of a variable</li></ul> |
 | type spread operator | [shipped](https://github.com/Microsoft/TypeScript/pull/28234) > 3.2rc | [shipped](https://github.com/facebook/flow/commit/ad443dc92879ae21705d4c61b942ba2f8ad61e4d) >=0.42 |
 | support for nullish coalescing proposal | no | yes |
 | support for decorators proposal | yes, legacy proposal | only parsing of legacy proposal, no type-checking |