Add useful code snippets for developers

cheatSheet.md

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+Useful References
+==================
+
+Find Module for PsiElement
+
+```java
+Module moduleForDomElement = ModuleUtil.findModuleForPsiElement(domElement.getXmlElement());
+Module moduleForPsiFile = ModuleUtil.findModuleForPsiElement(psiFile);
+```
+
+Find virtual files by type
+
+```java
+Collection<VirtualFile> virtualFiles = FileBasedIndex.getInstance().getContainingFiles(
+        FileTypeIndex.NAME, StdFileTypes.JAVA, GlobalSearchScope.moduleScope(moduleForPsiElement));
+```
+
+Get JavaPsiFile with Module scope from VirtualFile
+
+```java
+Collection<VirtualFile> virtualFiles = FileBasedIndex.getInstance().getContainingFiles(
+        FileTypeIndex.NAME, StdFileTypes.JAVA, GlobalSearchScope.moduleScope(module));
+
+VirtualFile virtualFile = virtualFiles.iterator().next();
+
+PsiFile file = PsiManager.getInstance(project)
+             .findFile(virtualFile);
+
+PsiJavaFile javaFile = (PsiJavaFile) file;
+```
+
+Find Java class by name, with specified scope
+
+```java
+final PsiClass psiClass = JavaPsiFacade.getInstance(module.getProject())
+        .findClass("fully.qualified.name.Foo", module.getModuleWithDependenciesAndLibrariesScope(true));
+```