[SYCL][Doc] Clean up contribution guide

- Limit lines to 80 characters
- Make links to the repo files relative
- Remove outdated information

Signed-off-by: Alexey Bader <[email protected]>

Author: bader

CONTRIBUTING.md

@@ -2,8 +2,7 @@
 
 ## License
 Intel Project for LLVM* technology is licensed under the terms of the
-Apache-2.0 with LLVM-exception license (
-[LICENSE.txt](https://github.com/intel/llvm/blob/intel/llvm/LICENSE.TXT))
+Apache-2.0 with LLVM-exception license ([LICENSE.txt](llvm/LICENSE.TXT))
 to ensure our ability to contribute this project to the LLVM project
 under the same license.
 
@@ -63,32 +62,38 @@ Then you just add a line to every git commit message:
 
 Use your real name (sorry, no pseudonyms or anonymous contributions.)
 
-If you set your `user.name` and `user.email` git configs, you can sign your commit automatically with `git commit -s`.
+If you set your `user.name` and `user.email` git configs, you can sign your
+commit automatically with `git commit -s`.
 
 ## Contribution process
 
 ### Development
 
- - Create a personal fork of the project on GitHub.
- - Use **sycl** branch as baseline for your changes.
- - Prepare your patch (follow [LLVM coding standards](https://llvm.org/docs/CodingStandards.html)).
- - Build the project and run all tests (see [GetStartedWithSYCLCompiler.md](https://github.com/intel/llvm/blob/sycl/sycl/doc/GetStartedWithSYCLCompiler.md))
+- Create a personal fork of the project on GitHub
+- Use **sycl** branch as baseline for your changes
+- Prepare your patch (follow
+  [LLVM coding standards](https://llvm.org/docs/CodingStandards.html))
+- Build the project and run all tests (see
+[GetStartedWithSYCLCompiler.md](sycl/doc/GetStartedWithSYCLCompiler.md))
 
 ### Review and acceptance testing
 
- - Create a pull request for your changes following [Creating a pull request instructions](https://help.github.com/articles/creating-a-pull-request/).
- - When the pull request is created signed-off check is done.
-     - **check_pr** - verifies that the signed-off mark is added to each commit message.
- - Once the pull request is approved by an Intel representative, build and functional testing are started.
-     - **sycl-ubu-x64-pr** - runs all LIT tests on machine with OpenCL runtimes for CPU and GPU.
- - Approval is lost once the PR branch is updated. New approval and checks rerun are required.
- - Once approval is received and all checks pass, the pull request is ready to be merged.
+- Create a pull request for your changes following [Creating a pull request
+instructions](https://help.github.com/articles/creating-a-pull-request/)
+- When the pull request is created signed-off check is done
+  - **check_pr** - verifies that the signed-off mark is added to each commit
+  message
+- Once the pull request is approved by an Intel representative, build and
+functional testing are done
+- Approval is reset once the PR branch is updated
+  - New approval and checks rerun are required
+- Once PR is approved and all checks pass, the pull request is ready for merge
 
 ### Merge
 
-Merge of pull request is done only by project maintainers. There are three options:
- - [Rebase and merge] The preferable choice for PRs containing a single commit.
- - [Squash and merge] Used when there are multiple commits in the PR. 
-   Squashing is done to shorten history and make sure that the project is buildable on any commit.
- - [Create a merge commit] Used for pull down PRs to avoid duplication of
-   LLVM commits.
+Project maintainers merge pull requests using one of the following options:
+- [Rebase and merge] The preferable choice for PRs containing a single commit
+- [Squash and merge] Used when there are multiple commits in the PR
+  - Squashing is done make sure that the project is buildable on any commit
+- [Create a merge commit] Used for LLVM pull-down PRs to preserve hashes of the
+commits pulled from the LLVM community repository