Add note about compiling artifacts to releasing steps (#1377)

* add note about compiling artifacts to release notes

* add explanation of truffle bug

RELEASING.md

@@ -34,6 +34,12 @@ git push upstream vX.Y.Z-rc.R
 
 Draft the release notes in our [GitHub releases](https://github.com/OpenZeppelin/openzeppelin-solidity/releases). Make sure to mark it as a pre-release! Try to be consistent with our previous release notes in the title and format of the text. Release candidates don't need a detailed changelog, but make sure to include a link to GitHub's compare page.
 
+Before publishing on npm you need to generate the build artifacts. This is not done automatically at the moment because of a bug in Truffle. Since some of the contracts should not be included in the package, this is a _hairy_ process that you need to do with care.
+
+1. Delete the `contracts/mocks` and `contracts/examples` directories.
+2. Run `truffle compile`. (Note that the Truffle process may never exit and you will have to interrupt it.)
+3. Recover the directories using `git checkout`. It doesn't matter if you do this now or later.
+
 Once the CI run for the new tag is green, publish on npm under the `next` tag.
 
 ```
@@ -62,6 +68,12 @@ git push upstream vX.Y.Z
 
 Draft the release notes in GitHub releases. Try to be consistent with our previous release notes in the title and format of the text. Make sure to include a detailed changelog.
 
+Before publishing on npm you need to generate the build artifacts. This is not done automatically at the moment because of a bug in Truffle. Since some of the contracts should not be included in the package, this is a _hairy_ process that you need to do with care.
+
+1. Delete the `contracts/mocks` and `contracts/examples` directories.
+2. Run `truffle compile`. (Note that the Truffle process may never exit and you will have to interrupt it.)
+3. Recover the directories using `git checkout`. It doesn't matter if you do this now or later.
+
 Once the CI run for the new tag is green, publish on npm.
 
 ```