更新 'README.md'

Signed-off-by: a1012112796 <1012112796@qq.com>

README.org

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+without latex delimiters the =_{i=1}= in =\sum_{i=1}^n a_n= is interpreted as subscript.
+we support =\(...\)=, =\[...\]=, =$$...$$= and =\begin{$env}...\end{$env}= as latex fragment delimiters.
+
+- \sum_{i=1}^n a_n (without latex delimiter)
+- \(\sum_{i=1}^n a_n\)
+- \[\sum_{i=1}^n a_n\]
+- $$\sum_{i=1}^n a_n$$
+- \begin{xyz}\sum_{i=1}^n a_n\end{xyz}
+- \begin{xyz}
+  \sum_{i=1}^n a_n
+  \end{xyz}
+- $2 + 2$, $3 - 3$

add.c

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+int add(int a, int b)
+{
+    return a + b;
+}