Installing Cypress guide improvment

closes #200, closes #218 and closes cypress-io/cypress#866

* npm install section clarification

- what is the npm package
- best practice highlighted

* direct download section precisions

- what is exactly downloaded as the user didn't knew it exactly

* new CDN Structure section

- CDN url pattern
- it give to the user enough informations to download Cypress in the wanted
version for the wanted platform (could download a linux Cypress version from
a windows workstation)
- example

* Advanced section

- warning added about CLI output when running cypress

source/guides/getting-started/installing-cypress.md

@@ -34,21 +34,43 @@ cd /your/project/path
 npm install cypress --save-dev
 ```
 
-This will install Cypress locally as a dev dependency for your project. We recommend this approach because:
+This will install Cypress locally as a dev dependency for your project.
+
+{% img /img/guides/installing-cli.gif %}
+
+{% note info %}
+Notice that the Cypress `npm` package is a wrapper around the Cypress binary. The binary downloaded by the `npm` package is the same as the binary downloaded directly as explained below.
+{% endnote %}
+
+{% note success %}
+{% fa fa-check-circle %} **Best Practice:** The recommended approach is to install Cypress with `npm` because :
 
 - Cypress is versioned like any other dependency.
 - It simplifies running Cypress in {% url 'Continuous Integration' continuous-integration %}.
-
-{% img /img/guides/installing-cli.gif %}
+{% endnote %}
 
 ## {% fa fa-download %} Direct Download
 
-If you're not using `node` or `npm` in your project or you just want to try Cypress out quickly, you can always {% url "download Cypress directly here" http://download.cypress.io/desktop %}.
+If you're not using `node` or `npm` in your project or you just want to try Cypress out quickly, you can always {% url "download Cypress directly from our CDN" http://download.cypress.io/desktop %}.
+
+It will download the latest available version. Your platform will be detected automatically.
 
 Just manually unzip and double click. Cypress will run without needing to install any dependencies.
 
 {% img /img/guides/installing-global.gif %}
 
+## {% fa fa-cloud-download %} CDN Structure
+
+If you want to download a specific Cypress version for a given platform, you can get it from our CDN.
+
+The url pattern is `https://cdn.cypress.io/desktop/<version>/<platform>/cypress.zip` where :
+- `<version>` is the wanted Cypress version number.
+- `<plaform>` is the wanted platform string identifier which could be `osx64`, `win64` or `linux64`.
+
+Example : 
+
+`https://cdn.cypress.io/desktop/1.0.1/win64/cypress.zip` will download Cypress `1.0.1` for Windows plaftorm.
+
 ## {% fa fa-refresh %} Continuous Integration
 
 Please read our {% url 'Continuous Integration' continuous-integration %} docs for help installing Cypress in CI. When running in linux you'll need to install some {% url 'system dependencies' continuous-integration#Dependencies %} or you can just use our {% url 'Docker images' docker-images %} which have everything you need prebuilt.
@@ -142,3 +164,7 @@ To override what is installed, you simply set `CYPRESS_BINARY_VERSION` with the
     ```
 
 In all cases, the fact that the binary was installed from a custom location *is not saved* in your `package.json` file. Every repeated installation would have to use the same environment variable to install the same binary.
+
+{% note warning %}
+If you have set an URL or a path to the `CYPRESS_BINARY_VERSION` environment variable, you will always get a warning message before tests execution. It is normal as the Cypress CLI cannot match the binary version with the `npm` package version. 
+{% endnote %}