Created directions on how release DevTool (#473)

* Created directions for release DevTools

tool/README.md

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+## How to release the next version of DevTools
+
+Create a branch for your release. Below we're creating release 0.0.15, with all the PRs. 
+
+## Update master branch and create a local release branch
+```shell
+cd ~/devtools-git/devtools
+
+checkout master
+
+git pull upstream master
+
+git checkout -b release_0_0_15
+
+cd packages/devtools
+```
+
+## Update the release number in two files:
+- **packages/devtools/pubspec.yaml**
+
+Change the ```version version: 0.0.14``` to ```version version: 0.0.15```
+
+- **packages/devtools/lib/devtools.dart**
+
+Change the ```const String version = '0.0.14';``` to ```const String version = '0.0.15';```
+
+## Update the CHANGELOG.md
+- **packages/devtools/CHANGELOG.md**
+
+Add the release number and date followed by the features or changes e.g.,
+
+```
+## 0.0.15 - 2019-04-01
+* Added a great feature ...
+```
+
+## Push the local branch
+
+```shell
+git add lib/devtools.dart 
+
+git add pubspec.yaml
+
+git add CHANGELOG.md
+
+git commit -a -m “Prepare for v0.0.15 release.”
+
+git push origin release_0_0_15
+```
+
+## Create the PR and Commit the above changes
+From the git UI tool create the PR, squash and commit.
+
+## Publishing DevTools
+### Update your master branch from the remote repository
+> Ensure that the tip of master is the above commit, just made with the exact set of PRs wanted.  Otherwise, checkout using the SHA1 of the above commit e.g.,
+``` git checkout -b release_15 <SHA1>``` then proceed to the step 'Prep to publish'.
+
+```shell
+cd ~/devtools-git/devtools
+
+git checkout master
+
+git pull upstream master
+```
+### Prep to publish
+```shell
+./tool/publish.sh
+``` 
+### Publish
+- #### Verify the package works (DevTools)
+- #### Publish the package
+```shell
+cd packages/devtools
+
+pub publish
+...
+Looks great! Are you ready to upload your package (y/n)? y
+```
+- #### Revert the change to .gitignore
+```shell
+git checkout .gitignore
+```
+- #### Create the tag for this release and push to the remote repository.
+```shell
+git tag -a v0.0.15 -m "DevTools 0.0.15"
+
+git push upstream v0.0.15
+```
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