added mermaid.md

mermaid.md

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+Let's see how is mermaid integrated into gitea:
+
+```mermaid
+graph TD;
+    A-->B;
+    A-->C;
+    B-->D;
+    C-->D;
+```
+
+```mermaid
+graph TD
+A[Christmas] -->|Get money| B(Go shopping)
+B --> C{Let me think}
+C -->|One| D[Laptop]
+C -->|Two| E[iPhone]
+C -->|Three| F[Car]
+```
+
+```mermaid
+sequenceDiagram
+    participant Alice
+    participant Bob
+    Alice->John: Hello John, how are you?
+    loop Healthcheck
+        John->John: Fight against hypochondria
+    end
+    Note right of John: Rational thoughts <br/>prevail...
+    John-->Alice: Great!
+    John->Bob: How about you?
+    Bob-->John: Jolly good!
+```
+
+```mermaid
+gantt
+        dateFormat  YYYY-MM-DD
+        title Adding GANTT diagram functionality to mermaid
+        section A section
+        Completed task            :done,    des1, 2014-01-06,2014-01-08
+        Active task               :active,  des2, 2014-01-09, 3d
+        Future task               :         des3, after des2, 5d
+        Future task2               :         des4, after des3, 5d
+        section Critical tasks
+        Completed task in the critical line :crit, done, 2014-01-06,24h
+        Implement parser and jison          :crit, done, after des1, 2d
+        Create tests for parser             :crit, active, 3d
+        Future task in critical line        :crit, 5d
+        Create tests for renderer           :2d
+        Add to mermaid                      :1d
+```