Add instruction to setup kokoro build jobs. (#114)

Author: rghetia

builds/Kokoro.md

@@ -0,0 +1,73 @@
+# Building on Kokoro
+
+Kokoro is build infrastructure based on jenkins. This document describes how to configure Kokoro
+builds for continuous integration as well as for pull requests.
+
+There are fours steps to have builds running on Kokoro.
+1. Job definition
+2. Build Configuration
+3. Build Script
+4. Automatic Trigger
+
+
+## Job Definition
+The job definition resides in Kokoro infrastructure. Refer to Kokoro
+documentation for details on job definition.
+
+### Example
+Here is a sample presubmit job defition for linux platform.
+* File Name: linux.cfg
+* File Content:
+```
+type: PRESUBMIT_GITHUB
+
+cluster: GCP_UBUNTU
+pool: "dynamic"
+
+scm {
+  github_scm {
+    owner: "census-instrumentation"
+    repository: "opencensus-java"
+    name: "opencensus-java"
+    presubmit_branch_regex: ".*"
+    build_config_dir: "buildscripts/kokoro"
+    commit_status_context: "Linux"
+    commit_status_url: LOGGING_URL
+    as_if_merged: true
+    use_webhook: true
+  }
+}
+```
+
+## Build Configuration
+The purpose of the build configuration is to tell kokoro how to build your
+project. It includes (but not limited to ) reference to buildscript location
+(build_file), build timeout value (timout_mins), etc.
+For detail description please refer to Kokoro documentation.
+
+The build configuration file resides in the directory specified by the field
+'build_config_dir' in job definition (see above). The name of the build
+configuration file should be same as the job definition.
+
+### Example
+File Name: linux.cfg
+File Content:
+
+```
+# Config file for internal CI
+
+# Location of the continuous shell script in repository.
+build_file: "opencensus-java/buildscripts/kokoro/linux.sh"
+timeout_mins: 60
+```
+
+## Build Script
+The buildscript is the script that runs to perform various build tasks.
+It is very specific to each project. 
+
+## Automatic Trigger
+In order to trigger an automatic build when pull request is created/updated
+(presubmit job) OR a commit is made (continuous integration job) Kokoro requires
+a webhook. Please add kokoro-team as admin of the repo and execute a command to
+create a webhook. Please refer to Kokoro documentation for this command.
+