explain how to install beta version of the test runner

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

@@ -194,6 +194,50 @@ CYPRESS_INSTALL_BINARY=0 npm install
 
 Now Cypress will skip its install phase once the npm module is installed.
 
+## Install beta version
+
+In very rare cases you might need to install a beta or pre-release version of Cypress to verify a fix that has been merged into the `develop` source branch, but has not been published yet. We build every commit in {% url "`develop` branch" https://github.com/cypress-io/cypress/commits/develop %} for each platform and test it against downstream projects. For example, in the image below the last commit has the short SHA `e5106d9`:
+
+{% imgTag /img/guides/install/last-commit.png "Last commit on develop branch" %}
+
+The simplest way to find beta version of Cypress Test Runner for this commit is to look at the commits for the test project {% url "cypress-test-example-repos" https://github.com/cypress-io/cypress-test-example-repos/commits/master %}. You will see individual commits for each built platform and architecture: `darwin`, `linux`, `win 32bit` and `win 64bit`. The built commit SHA `e5106d9` is in the subject line of the test commit:
+
+{% imgTag /img/guides/install/test-commits.png "Test commit per platform" %}
+
+These beta builds are platform-specific. Pick the platform that matches yours; for example I am on Mac, thus I will click on the commit "Testing new darwin x64 ...". This commit has a custom message that shows special temporary URL of the built binary for Mac OS and the matching npm `cypress` package.
+
+{% imgTag /img/guides/install/beta-binary.png "Beta binary information" %}
+
+To install this beta binary on Mac I need to set the `CYPRESS_INSTALL_BINARY` environment variable to the shown `https://cdn.cypress.io/beta/binary/.../cypress.zip` value and run `npm install https://cdn.cypress.io/beta/npm/3.3.2/.../cypress.tgz`. The command in the terminal will be:
+
+```shell
+export CYPRESS_INSTALL_BINARY=https://cdn.cypress.io/beta/binary/3.3.2/darwin-x64/circle-develop-e5106d95f51eec477b8e66609939979fb87aab56-126014/cypress.zip
+npm install https://cdn.cypress.io/beta/npm/3.3.2/circle-develop-e5106d95f51eec477b8e66609939979fb87aab56-126013/cypress.tgz
+```
+
+If my machine is Windows 64bit, I will click on "Testing new win32 x64 ..." commit and run
+
+```shell
+set CYPRESS_INSTALL_BINARY=https://cdn.cypress.io/beta/binary/3.3.2/win32-x64/appveyor-develop-e5106d95f51eec477b8e66609939979fb87aab56-25451270/cypress.zip
+npm install https://cdn.cypress.io/beta/npm/3.3.2/appveyor-develop-e5106d95f51eec477b8e66609939979fb87aab56-25451270/cypress.tgz
+```
+
+On Linux CI you should install the beta binary from the "Testing new linux x64 ..." commit
+
+```shell
+export CYPRESS_INSTALL_BINARY=https://cdn.cypress.io/beta/binary/3.3.2/linux-x64/circle-develop-e5106d95f51eec477b8e66609939979fb87aab56-125973/cypress.zip
+npm install https://cdn.cypress.io/beta/npm/3.3.2/circle-develop-e5106d95f51eec477b8e66609939979fb87aab56-125992/cypress.tgz
+```
+
+### Beta binary URL format
+
+The above `CYPRESS_INSTALL_BINARY` urls are temporary - they are purged after 30 days. The format of the url is as follows:
+
+```text
+https://cdn.cypress.io/beta/binary/<version>/<platform>-<arch>/
+<ci name>-<branch name>-<full commit SHA>-<CI build number>/cypress.zip
+```
+
 ## Binary cache
 
 As of version `3.0`, Cypress downloads the matching Cypress binary to the global system cache, so that the binary can be shared between projects. By default, global cache folders are: