Document how to force linking with static libraries

Reviewed-by: Rich Salz <rsalz@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/1889)

Configurations/README

diff --git a/Configurations/README b/Configurations/README
index 0b82ded..21a6f03 100644
--- a/Configurations/README
+++ b/Configurations/README
@@ -339,6 +339,17 @@
 to be located in the source tree while files given through DEPEND are
 expected to be located in the build tree)
 
+It's also possible to depend on static libraries explicitely:
+
+    DEPEND[foo]=libsomething.a
+    DEPEND[libbar]=libsomethingelse.a
+
+This should be rarely used, and care should be taken to make sure it's
+only used when supported.  For example, native Windows build doesn't
+support build static libraries and DLLs at the same time, so using
+static libraries on Windows can only be done when configured
+'no-shared'.
+
 For some libraries, we maintain files with public symbols and their
 slot in a transfer vector (important on some platforms).  It can be
 declared like this: