Add engine contrib info.

[skip ci]

CONTRIBUTING.adoc

@@ -1,4 +1,4 @@
-// Copyright 2019 Rene Rivera
+// Copyright 2019-2020 Rene Rivera
 // Copyright 2003, 2006 Vladimir Prus
 // Distributed under the Boost Software License, Version 1.0.
 // (See accompanying file LICENSE.txt or http://www.boost.org/LICENSE_1_0.txt)
@@ -150,3 +150,30 @@ else
     #
 }
 ----
+
+== ENGINE
+
+Developing in the `b2` engine, the C++ part, requires two steps to be
+effective: building the "stable" engine, and developing the
+"in-progress" engine.
+
+What is the "stable" engine is up to you. It only refers to a build of the
+engine you know is at a good working state. When you are at a point the
+source is stable you can run `bootstrap.sh/bat` from the root. That will
+create the `b2` executable at the root. You can then use this version to run
+regular B2 builds as needed both within the B2 tree and in other projects.
+
+The "in-progress" engine is whatever build you happen to be testing at the
+moment. There are two ways to build this be engine. You can either
+(a) run `b2 b2` at the root, or (b) run `build.sh/bat` in `src/engine`.
+
+Using (a) will place, by default, a fully debuggable `b2` in the `.build`
+directories. You can run that one from a debugger with full symbols and
+stepping features. This should be the first choice in developing in the
+engine.
+
+After using (a) to implement functionality you can use (b) to fully test
+that functionality. The engine built from (b) is fully optimized and
+is the one used, by default, by the test system when running in the `test`
+directory. Before submitting patches it's required to build this way and
+run the tests in at least one toolset version (but preferably at least two).