Update README.md

Author: Oldes

README.md

@@ -12,6 +12,30 @@ but keep the source code clean and project easy to build. Use [CHANGES.md](https
 
 Preferred way for issue reporting is using [dedicated issue repository](https://github.com/Oldes/Rebol-issues/issues). It's a fork of the original Rebol issue repository, which was filled with issues from [CureCode issue tracker](https://www.curecode.org/rebol3/view-tickets.rsp), which was used before Rebol was on Github. I'm not using the original Rebol issue repository, because I was not allowed to even add labels to my own issues. It was later moved under Metaeducation account and is used for Ren-C development anyway.
 
+### Precompiled binaries
+
+There are available precompiled binaries for each [release](https://github.com/Oldes/Rebol3/releases). So far there are 3 main build types:
+1. **Base** is a build with minimal additions (not much useful)
+2. **Core** includes a little bit more stuff than the **Base**
+3. **Bulk** is a build which includes almost everything.
+
+And there is also the Host exe and the DLL - the Rebol library is separated and used from the host application. That is from times before open sourcing Rebol completely. Only host part was open and the library was still closed. In theory you can have one library and many tiny host applications. I'm building just the Core on Windows so far to see, if it is still working.
+
+### Building Rebol
+
+Rebol itself is not a compiler (like [Red language](https://www.red-lang.org/)) but just an interpreter. You must have some compiler of your choice to compile Rebol. For Windows you may want to use any of these:
+1. [Microsoft Visual Studio](https://visualstudio.microsoft.com/)
+2. [CLANG](https://clang.llvm.org/)
+3. [GCC (Mingw)](https://www.mingw-w64.org/)
+
+Once you have any of these compilers, you must use [Siskin Builder tool](https://github.com/Siskin-framework/Builder/releases), which is actually a customised Rebol needed to [preprocess it's own sources](https://github.com/Oldes/Rebol3/blob/607572d5485f2d8e44aeea4ffadabf0c7374eee5/make/rebol3.nest#L981). It deserves own documentation as it's not mean to be used only to build Rebol. Meanwhile you may take a look at the source of the Github [workflow file for building all Rebol variants](https://github.com/Oldes/Rebol3/blob/master/.github/workflows/build-all.yml) used in the releases.
+
+For a local use, you just do: `siskin <SOME-NEST-FILE>`, which starts CLI in an interactive mode.
+
+The Siskin builder itself may be used as an example, how to build a custom utility based on Rebol sources.
+The specification is [defined in this `*.nest` file](https://github.com/Siskin-framework/Builder/blob/master/tree/rebol/siskin.nest), where important is [`CUSTOM_STARTUP` define](https://github.com/Siskin-framework/Builder/blob/756d9531e2f461c22d626ca5458dad4e0c8bd3cd/tree/rebol/siskin.nest#L36) and [some files](https://github.com/Siskin-framework/Builder/blob/756d9531e2f461c22d626ca5458dad4e0c8bd3cd/tree/rebol/siskin.nest#L22-L30), which should be included and [some optional Rebol parts](https://github.com/Siskin-framework/Builder/blob/756d9531e2f461c22d626ca5458dad4e0c8bd3cd/tree/rebol/siskin.nest#L14-L18) if needed.
+
+
 ### Screenshots
 
 ![](https://github.com/Oldes/media/blob/master/screens/rebol-windows-terminal.PNG?raw=true "Rebol in Windows Terminal")