Add docs for main_repo (#4827)

Fixes https://github.com/google/oss-fuzz/issues/4825

docs/getting-started/new_project_guide.md

@@ -77,7 +77,8 @@ This configuration file stores project metadata. The following attributes are su
 - [homepage](#homepage)
 - [language](#language)
 - [primary_contact](#primary)
-- [auto_ccs](#primary)
+- [auto_ccs](#auto_ccs)
+- [main_repo](#main_repo)
 - [vendor_ccs](#vendor) (optional)
 - [sanitizers](#sanitizers) (optional)
 - [architectures](#architectures) (optional)
@@ -101,6 +102,9 @@ Programming language the project is written in. Values you can specify include:
 The primary contact and list of other contacts to be CCed. Each person listed gets access to ClusterFuzz, including crash reports and fuzzer statistics, and are auto-cced on new bugs filed in the OSS-Fuzz
 tracker. If you're a primary or a CC, you'll need to use a [Google account](https://support.google.com/accounts/answer/176347?hl=en) to get full access. ([why?]({{ site.baseurl }}/faq/#why-do-you-require-a-google-account-for-authentication)).
 
+### main_repo {#main_repo}
+Path to source code repository hosting the code, e.g. `https://path/to/main/repo.git`. 
+
 ### vendor_ccs (optional) {#vendor}
 The list of vendor email addresses that are downstream consumers of the project and want access to
 the bug reports as they are filed.