Update glossary.md

Commit migrated from dotnet/coreclr@c28e5e8

docs/coreclr/project-docs/glossary.md

@@ -15,6 +15,7 @@ As much as possible, we should link to the most authoritative and recent source
 * JIT: [Just-in-Time](https://github.com/dotnet/coreclr/blob/master/Documentation/botr/ryujit-overview.md) compiler. RyuJIT is the code name for the next generation Just-in-Time(aka "JIT") for the .NET runtime.
 * LCG: Lightweight Code Generation. An early name for [dynamic methods](https://github.com/dotnet/coreclr/blob/master/src/mscorlib/src/System/Reflection/Emit/DynamicMethod.cs).
 * NGen: Native Image Generator.
+* NYI: Not Yet Implemented
 * PAL: [Platform Adaptation Layer](http://archive.oreilly.com/pub/a/dotnet/2002/03/04/rotor.html). Provides an abstraction layer between the runtime and the operating system
 * PE: Portable Executable.
 * ProjectN: Codename for the first version of [.NET Native for UWP](https://msdn.microsoft.com/en-us/vstudio/dotnetnative.aspx).