update conventions and guidelines about C++ standard and requirements

doc/github-development-workflow.md

@@ -136,7 +136,8 @@ Here are some items to check:
     * string.h -> cstring
     * time.h -> ctime
     * Do NOT replace (as they are C++-11): `inttypes.h` and `stdint.h`.
-  * Code should follow the C++-98 standard. C++-11 is only accepted
+  * Code must follow the C++-11 standard. C++98-only is no longer accepted
+  * Code should use `nullptr` instead of `NULL` where applicable.
   in individual special purpose packages
   * indentation is 2 spaces per level
   * there should be NO tabs and no trailing whitespace (review the "checkstyle" test on pull requests)
@@ -145,6 +146,8 @@ Here are some items to check:
   Forward declarations should be used instead when possible.
   * iostreams should be avoided. LAMMPS uses stdio from the C-library.
   * use of STL in headers and class definitions should be avoided.
+  exception is <string>, but it won't need to be explicitly included
+  since pointers.h already includes it. so std::string can be used directly.
   * there MUST NOT be any "using namespace XXX;" statements in headers.
   * static class members should be avoided at all cost.
   * anything storing atom IDs should be using `tagint` and not `int`.
@@ -152,6 +155,8 @@ Here are some items to check:
   compiling LAMMPS with `-DLAMMPS_BIGBIG`.
   * when including both `lmptype.h` (and using defines or macros from it)
   and `mpi.h`, `lmptype.h` must be included first.
+  * see https://github.com/lammps/lammps/blob/master/doc/include-file-conventions.md
+  for general include file conventions and best practices
   * when pair styles are added, check if settings for flags like
   `single_enable`, `writedata`, `reinitflag`, `manybody_flag`
   and others are correctly set and supported.