README: Document how to enable/build external dependencies.

README.md

@@ -94,9 +94,35 @@ Browse available modules on
 Standard library modules come from
 [micropython-lib](https://github.com/micropython/micropython-lib) project.
 
-(*) Debian/Ubuntu/Mint derivative Linux distros will require build-essentials,
-libffi-dev and pkg-config packages installed. If you have problems with some
-dependencies, they can be disabled in unix/mpconfigport.mk .
+External dependencies
+---------------------
+
+Building Unix version requires some dependencies installed. For
+Debian/Ubuntu/Mint derivative Linux distros, install `build-essentials`
+(includes toolchain and make), `libffi-dev`, and `pkg-config` packages.
+
+Other dependencies can be built together with MicroPython. Oftentimes,
+you need to do this to enable extra features or capabilities. To build
+these additional dependencies, first fetch git submodules for them:
+
+    $ git submodule update --init
+
+Use this same command to get the latest versions of dependencies, as
+they are updated from time to time. After that, in `unix/` dir, execute:
+
+    $ make deplibs
+
+This will build all available dependencies (regardless whether they
+are used or not). If you intend to build MicroPython with additional
+options (like cross-compiling), the same set of options should be passed
+to `make deplibs`. To actually enabled use of dependencies, edit
+`unix/mpconfigport.mk` file, which has inline descriptions of the options.
+For example, to build SSL module (required for `upip` tool described above),
+set `MICROPY_PY_USSL` to 1.
+
+In `unix/mpconfigport.mk`, you can also disable some dependencies enabled
+by default, like FFI support, which requires libffi development files to
+be installed.
 
 The STM version
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