add readme

Author: sjjian

README.md

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+# Oracle SQL Parser
+this is an oracle sql parser
+
+## supported statement
+|statement| sub statement |yacc|ast|
+|----|----|----|----|
+|Alter table|Add column| yes|yes|
+|Alter table|Modify column| yes|no|
+|Alter table|Drop column| yes|no|
+|Alter table|Rename column| yes|no|
+
+## usage
+```go
+
+```