add section about running test files

source/guides/core-concepts/writing-and-organizing-tests.md

@@ -391,7 +391,7 @@ The code above will produce a suite with 4 tests:
 
 ## Assertion Styles
 
-Cypress supports both BDD (`expect`/`should`) and TDD (`assert`) style assertions. {% url "Read more about assertions." assertions %}
+Cypress supports both BDD (`expect`/`should`) and TDD (`assert`) style plain assertions. {% url "Read more about plain assertions." assertions %}
 
 ```javascript
 it('can add numbers', () => {
@@ -403,6 +403,39 @@ it('can subtract numbers', () => {
 })
 ```
 
+The {% url "`.should()`" should %} command and its alias {% url "`.and()`" and %} can also be used to more easily chain assertions off of Cypress commands. {% url "Read more about assertions." introduction-to-cypress#Assertions %}
+
+```js
+cy.wrap(add(1, 2)).should('equal', 3)
+```
+
+# Running tests
+
+## Run a single spec file
+
+We suggest running test files individually by clicking on the spec filename to ensure the best performance. For example the {% url "Cypress RealWorld App" https://github.com/cypress-io/cypress-example-realworld %} has multiple test files, but below we run a single "new-transaction.spec.ts" test file.
+
+{% imgTag /img/guides/core-concepts/run-single-spec.gif "Running a single spec" %}
+
+## Run all specs
+
+You can run all spec files together by clicking the "Run all specs" button. This mode is equivalent to concatenating all spec files together into a single piece of test code.
+
+{% imgTag /img/guides/core-concepts/run-all-specs.gif "Running all specs" %}
+
+{% note danger %}
+{% fa fa-warning %} Be wary of root-level hooks, as they could execute in a surprising order when clicking the "Run all specs" button. Instead place them inside `describe` or `context` suites for isolation. Read {% url "'Be careful when running all specs together'" https://glebbahmutov.com/blog/run-all-specs/ %}.
+{% endnote %}
+
+## Run filtered specs
+
+You can also run a subset of all specs by entering a text search filter. Only the specs with relative file paths containing the search filter will remain and be run as if concatenating all spec files together when clicking the "Run N specs" button.
+
+- The search filter is case-insensitive; the filter "ui" will match both "UI-spec.js" and "admin-ui-spec.js" files.
+- The search filter is applied to the entire relative spec file path, thus you can use folder names to limit the specs; the filter "ui" will match both "admin-ui.spec.js" and "ui/admin.spec.js" files.
+
+{% imgTag /img/guides/core-concepts/run-selected-specs.gif "Running specs matching the search filter" %}
+
 # Watching tests
 
 When running in using {% url "`cypress open`" command-line#cypress-open %}, Cypress watches the filesystem for changes to your spec files. Soon after adding or updating a test Cypress will reload it and run all of the tests in that spec file.