Document merge process for dmd, druntime and phobos.

gcc/d/ChangeLog:

	* README.gcc: New file.

libphobos/ChangeLog:

	* README.gcc: New file.

From-SVN: r267885

gcc/d/ChangeLog

+2019-01-12  Iain Buclaw  <ibuclaw@gdcproject.org>
+
+	* README.gcc: New file.
+
 2019-01-01  Jakub Jelinek  <jakub@redhat.com>
 
 	Update copyright years.

gcc/d/README.gcc

+The files in the dmd subdirectory are part of the front-end for the
+Digital Mars D compiler, hosted at https://github.com/dlang/dmd/.
+
+They cover the lexical analysis, parsing, and semantic analysis of the
+D Programming Language defined in the documents at https://dlang.org/.
+
+To report a problem or look up known issues with the dmd front-end,
+please visit the issue tracker at https://issues.dlang.org/.
+
+All changes to dmd should go through the upstream repository first,
+then merged back to GCC.

libphobos/ChangeLog

+2019-01-12  Iain Buclaw  <ibuclaw@gdcproject.org>
+
+	* README.gcc: New file.
+
 2019-01-01  Jakub Jelinek  <jakub@redhat.com>
 
 	Update copyright years.

libphobos/README.gcc

+The files in this directory where noted are part of the DRuntime
+and Phobos library.
+
+DRuntime is the low-level runtime library backing the D programming
+language, hosted at https://github.com/dlang/druntime/.
+
+Phobos is the standard library for the D Programming Language, hosted
+at https://github.com/dlang/phobos/.
+
+The following sources and directories are part of DRuntime:
+  libdruntime/core/
+  libdruntime/gc/
+  libdruntime/gcstub/
+  libdruntime/object.d
+  libdruntime/rt/
+
+The following sources and directories are part of Phobos:
+  src/etc/
+  src/index.d
+  src/std/
+
+To report a bug or look up known issues with the runtime or standard
+library please visit the issue tracker at https://issues.dlang.org/.
+
+All changes to either of these libraries should go through the
+upstream repository first, then merged back to GCC.