Add notes about latex fragments.

The places they are currently occurring in my posts are unintentional, so I don't think I will be implementing latex fragments right now, but the notes are worth keeping.

org_test_documents/latex_fragment/notes.org

@@ -0,0 +1,36 @@
+* Double dollar
+#+begin_src org
+  $$CONTENTS$$
+#+end_src
+becomes
+#+begin_src text
+  \[CONTENTS\]
+#+end_src
+
+#+begin_src org
+  $$1+1=2$$
+#+end_src
+
+#+begin_src text
+  \[1+1=2\]
+#+end_src
+
+This gets interpreted by mathjax.
+* Single Dollar
+#+begin_src org
+  PRE$BORDER1 BODY BORDER2$POST
+#+end_src
+becomes
+#+begin_src text
+  \(BORDER1 BODY BORDER2\)
+#+end_src
+
+#+begin_src org
+  foo$a bar b$.
+#+end_src
+becomes
+#+begin_src text
+  foo\(a bar b\).
+#+end_src
+
+This gets interpreted by mathjax.