add Issue and Pull Request templates

See https://github.com/mxe/mxe/issues/1235

.github/ISSUE_TEMPLATE

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+If a package fails to build, upload the log file to
+https://gist.github.com/ and put the link here.

.github/PULL_REQUEST_TEMPLATE.md

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+Please read http://mxe.cc/#creating-packages
+
+In particular, make sure that your build rules:
+
+  * install .pc file,
+  * install bin/test-pkg.exe compiled with flags by pkg-config,
+  * install .dll to bin/ and .a, .dll.a to lib/,
+  * use $(TARGET)-cmake instead of cmake,
+  * build in `$(BUILD_DIR)` instead of `$(SOURCE_DIR)`,
+  * do not run target executables with Wine,
+  * do not download anything while building,
+  * do not install documentation,
+  * do not install .exe files except test and build systems,
+
+and .patch files are generated by tools/patch-tool-mxe.
+
+If you add a package, you can use tool tools/skeleton.py.
+
+Thanks!