Add some developer documentation.

* DEPENDENCIES: New file.
* HACKING: New file.
* Makefile.in (SOURCE_FILES): Add them.

DEPENDENCIES

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+The following packages should be installed before GNU gperf is installed
+(runtime dependencies that are also build dependencies): None.
+
+
+The following packages should be installed when GNU gperf is installed
+(runtime dependencies, but not build dependencies):
+
+* The Gnulib localizations.
+  + Recommended.
+    Needed for localization to the user's language.
+  + Documentation:
+    https://www.gnu.org/software/gnulib/manual/html_node/Localization.html
+  + Download:
+    https://ftp.gnu.org/gnu/gnulib/gnulib-l10n-*
+
+
+The following should be installed when GNU gperf is built, but are not
+needed later, once it is installed (build dependencies, but not runtime
+dependencies):
+
+* A C and C++ runtime, compiler, linker, etc.
+  + Mandatory.
+    Either the platform's native 'cc' and 'c++', or GCC 3.1 or newer.
+  + GCC Homepage:
+    https://gcc.gnu.org/
+  + Download:
+    https://ftp.gnu.org/gnu/gcc/
+
+* A 'make' utility.
+  + Mandatory.
+    Either the platform's native 'make' (for in-tree builds only),
+    or GNU Make 3.79.1 or newer.
+  + GNU Make Homepage:
+    https://www.gnu.org/software/make/
+  + Download:
+    https://ftp.gnu.org/gnu/make/
+
+* A shell
+  + Mandatory.
+    Either the platform's native 'sh', or Bash.
+  + Homepage:
+    https://www.gnu.org/software/bash/
+  + Download:
+    https://ftp.gnu.org/gnu/bash/
+
+* Core POSIX utilities, including:
+    [ basename cat chgrp chmod chown cp dd echo expand expr
+    false hostname install kill ln ls md5sum mkdir mkfifo
+    mknod mv printenv pwd rm rmdir sleep sort tee test touch
+    true uname
+  + Mandatory.
+    Either the platform's native utilities, or GNU coreutils.
+  + Homepage:
+    https://www.gnu.org/software/coreutils/
+  + Download:
+    https://ftp.gnu.org/gnu/coreutils/
+
+* The comparison utilities 'cmp' and 'diff'.
+  + Mandatory.
+    Either the platform's native utilities, or GNU diffutils.
+  + Homepage:
+    https://www.gnu.org/software/diffutils/
+  + Download:
+    https://ftp.gnu.org/gnu/diffutils/
+
+* Grep.
+  + Mandatory.
+    Either the platform's native grep, or GNU grep.
+  + Homepage:
+    https://www.gnu.org/software/grep/
+  + Download:
+    https://ftp.gnu.org/gnu/grep/
+
+* Awk.
+  + Mandatory.
+    Either the platform's native awk, mawk, or nawk, or GNU awk.
+  + Homepage:
+    https://www.gnu.org/software/gawk/
+  + Download:
+    https://ftp.gnu.org/gnu/gawk/