doc: add instructions to run checkbuild before PR

We will ask contributors to check the PR builds correctly on all
architectures before submitting the review.

Signed-off-by: Riccardo Mancini <[email protected]>

Author: Manciukic

.github/pull_request_template.md

@@ -16,6 +16,8 @@ Certificate of Origin and signing off your commits, please check
 ## PR Checklist
 
 - [ ] I have read and understand [CONTRIBUTING.md][3].
+- [ ] I have run `tools/devtool checkbuild --all` to verify that the PR passes
+  build checks on all supported architectures.
 - [ ] I have run `tools/devtool checkstyle` to verify that the PR passes the
   automated style checks.
 - [ ] I have described what is done in these changes, why they are needed, and

CONTRIBUTING.md

@@ -63,13 +63,16 @@ following in the Firecracker root directory:
 ```
 cat >> .git/hooks/pre-commit << EOF
 ./tools/devtool checkstyle || exit 1
+./tools/devtool checkbuild --all || exit 1
 EOF
 ```
 
-This will automatically lint your Rust, markdown and python changes when running
-`git commit`, as well as running any other checks our CI validates as part of
-its 'Style' step. Most reported violations can be automatically fixed using
-`./tools/devtool fmt`.
+The first command will automatically lint your Rust, markdown and python changes
+when running `git commit`, as well as running any other checks our CI validates
+as part of its 'Style' step. Most reported violations can be automatically fixed
+using `./tools/devtool fmt`. The second command will then check that the code
+correctly compiles on all supported architectures, and that it passes Rust
+clippy rules defined for the project.
 
 Your contribution needs to meet the following standards: