added doc/markdown.md to start my own markdown cheat sheet

docs/markdown.md

@@ -0,0 +1,15 @@
+# markdown
+
+## links
+
+To insert links directly in the text use:
+```markdown
+[display name](url-or-link-to-markdown-anker)
+```
+Links can be defined in a separate (not rendered) section as follows:
+```markdown
+[display name]: url-or-link-to-markdown-anker
+
+Reference the Link in text with [display name].
+```
+In the last example the Link can be defined after the usage.