Add steps to build Thanos project for contributors (thanos-io#401)

* add steps to build thanos project

* add more details about git workflow

* typo

Author: yuecong

CONTRIBUTING.md

@@ -10,15 +10,36 @@ Please follow the [code of conduct](CODE_OF_CONDUCT.md) in all your interactions
 
 1. Read [getting started docs](docs/getting_started.md) and prepare Thanos.
 2. Familarize yourself with [Makefile](Makefile) commands like `format`, `build`, `proto` and `test`.
-3. Keep PRs as small as possible. Chain them if needed (base PR on other PRs).
-4. If you don't have a live object store ready add these envvars to skip tests for these:
+3. Fork improbable-eng/thanos.git and start development from your own fork. Here are sample steps to setup your development environment:
+```console
+$ mkdir -p $GOPATH/src/github.com/improbable-eng
+$ cd $GOPATH/src/github.com/improbable-eng
+$ git clone https://github.com/<your_github_id>/thanos.git
+$ cd thanos
+$ git add remote upstream https://github.com/improbable-eng/thanos.git
+$ git remote update
+$ git merge upstream/master
+$ make build
+$ ./thanos -h
+```
+4. Keep PRs as small as possible. For each of your PR, you create one branch based on the latest master. Chain them if needed (base PR on other PRs). Here are sample steps you can follow. You can get more details about the workflow from [here](https://gist.github.com/Chaser324/ce0505fbed06b947d962).
+```console
+$ git checkout master
+$ git remote update
+$ git merge upstream/master
+$ git checkout -b <your_branch_for_new_pr>
+$ make build
+$ <Iterate your development>
+$ git push origin <your_branch_for_new_pr>
+```
+5. If you don't have a live object store ready add these envvars to skip tests for these:
 - THANOS_SKIP_GCS_TESTS to skip GCS tests.
 - THANOS_SKIP_S3_AWS_TESTS to skip AWS tests.
 
 If you skip both of these, the store specific tests will be run against memory object storage only.
 CI runs GCS and inmem tests only for now. Not having these variables will produce auth errors against GCS or AWS tests.
 
-5. If your change affects users (adds or removes feature) consider adding the item to [CHANGELOG](CHANGELOG.md)
-6. You may merge the Pull Request in once you have the sign-off of at least one developers with write access, or if you
+6. If your change affects users (adds or removes feature) consider adding the item to [CHANGELOG](CHANGELOG.md)
+7. You may merge the Pull Request in once you have the sign-off of at least one developers with write access, or if you
    do not have permission to do that, you may request the second reviewer to merge it for you.
-7. If you feel like your PR waits too long for a review, feel free to ping [`#thanos-dev`](https://join.slack.com/t/improbable-eng/shared_invite/enQtMzQ1ODcyMzQ5MjM4LWY5ZWZmNGM2ODc5MmViNmQ3ZTA3ZTY3NzQwOTBlMTkzZmIxZTIxODk0OWU3YjZhNWVlNDU3MDlkZGViZjhkMjc) channel on our slack for review!
+8. If you feel like your PR waits too long for a review, feel free to ping [`#thanos-dev`](https://join.slack.com/t/improbable-eng/shared_invite/enQtMzQ1ODcyMzQ5MjM4LWY5ZWZmNGM2ODc5MmViNmQ3ZTA3ZTY3NzQwOTBlMTkzZmIxZTIxODk0OWU3YjZhNWVlNDU3MDlkZGViZjhkMjc) channel on our slack for review!