upload an introduction about chemical formula and update readme.md (#489)

* upload an introduction about chemical formula

* rename 2 files

* update readme.md at TODO in chemstery

* rename 2 files and update readme.md at TODO in chemstery

* update README_zh-CN.md at TODO in chemstery

README.md

@@ -325,7 +325,7 @@ TODO
 - [ ] Code block recognition within the text
 - [ ] Table of contents recognition
 - [x] Table recognition
-- [ ] Chemical formula recognition
+- [ ] [Chemical formula recognition](docs/chemical_knowledge_introduction/introduction.pdf)
 - [ ] Geometric shape recognition
 
 # Known Issues

README_zh-CN.md

@@ -329,7 +329,7 @@ TODO
 - [ ] 正文中代码块识别
 - [ ] 目录识别
 - [x] 表格识别
-- [ ] 化学式识别
+- [ ] [化学式识别](docs/chemical_knowledge_introduction/introduction.pdf)
 - [ ] 几何图形识别
 
 # Known Issues