README: make note of what features are missing without LLVM

Author: andrewrk

README.md

@@ -66,20 +66,38 @@ page on the wiki.
 
 ## Building from Source without LLVM
 
-If you don't need your Zig compiler to have LLVM extensions enabled, you can
-follow these instructions instead.
-
 In this case, the only system dependency is a C compiler.
 
 ```
 cc -o bootstrap bootstrap.c
 ./bootstrap build
 ```
 
-You can pass any options to this that you would pass to `zig build`.
+You can pass any options to this that you would pass to `zig build` (see
+`--help` for options).
+
+[Without LLVM extensions](https://github.com/ziglang/zig/issues/16270), a Zig
+compiler is missing these features:
+- Release mode optimizations
+- aarch64 machine code backend
+- `@cImport` / `zig translate-c`
+- Ability to compile C files
+- Ability to compile assembly files
+- [Some ELF linking features](https://github.com/ziglang/zig/issues/17749)
+- [Most COFF/PE linking features](https://github.com/ziglang/zig/issues/17751)
+- [Some WebAssembly linking features](https://github.com/ziglang/zig/issues/17750)
+- [COFF linking](https://github.com/ziglang/zig/issues/17751)
+- [Ability to output LLVM bitcode](https://github.com/ziglang/zig/issues/13265)
+- [Windows resource file compilation](https://github.com/ziglang/zig/issues/17752)
+- [Ability to create import libs from def files](https://github.com/ziglang/zig/issues/17807)
+- [Automatic importlib file generation for Windows DLLs](https://github.com/ziglang/zig/issues/17753)
+- [Ability to create static archives from object files](https://github.com/ziglang/zig/issues/9828)
+- Ability to compile C++, Objective-C, and Objective-C++ files
 
 ## Contributing
 
+[Donate monthly](https://ziglang.org/zsf/).
+
 Zig is Free and Open Source Software. We welcome bug reports and patches from
 everyone. However, keep in mind that Zig governance is BDFN (Benevolent
 Dictator For Now) which means that Andrew Kelley has final say on the design