Port contents of Documentation/PackageSecurity.md to DocC (#8726)

Fixes #8591 

Ports the contents of `Documentation/PackageSecurity.md` to our DocC
catalog in `Sources/PackageManagerDocs/PackageSecurity.md`.

Author: bripeticca

Sources/PackageManagerDocs/Documentation.docc/Documentation.md

@@ -16,6 +16,7 @@ The Swift Package Manager leets you share your code as a package, depend on and
 
 - <doc:GettingStarted>      <!-- tutorial or article based walk through -->
 - <doc:IntroducingPackages>
+- <doc:PackageSecurity>
 
 ### Guides
 

Sources/PackageManagerDocs/Documentation.docc/PackageSecurity.md

@@ -0,0 +1,32 @@
+# Package Security
+
+Learn about the security features that the package manager implements.
+
+## Trust on First Use
+
+The package manager records **fingerprints** of downloaded package versions so that
+it can perform [trust-on-first-use](https://en.wikipedia.org/wiki/Trust_on_first_use)
+(TOFU). 
+That is, when a package version is downloaded for the first time, the package manager trusts that 
+it has downloaded the correct contents and requires subsequent downloads of the same 
+package version to have the same fingerprint. 
+If the fingerprint changes, it might be an indicator that the package has been
+compromised and the package manager either warns or returns an error.
+
+Depending on where a package version is downloaded from, a different value is
+used as its fingerprint:
+                             
+| Package Version Origin | Fingerprint |
+| ---------------------- | ----------- |
+| Git repository         | Git hash of the revision |
+| Package registry       | Checksum of the source archive |
+
+The package manager keeps version fingerprints for each package in a single file
+under the `~/.swiftpm/security/fingerprints` directory.
+  - For a Git repository package, the fingerprint filename takes the form of `{PACKAGE_NAME}-{REPOSITORY_URL_HASH}.json` (such as `LinkedList-5ddbcf15.json`).
+  - For a registry package, the fingerprint filename takes the form of `{PACKAGE_ID}.json` (such as `mona.LinkedList.json`).
+
+For packages retrieved from a registry, the package manager expects all registries to provide consistent fingerprints for packages they host.
+If registries have conflicting fingerprints, package manager reports that as an error.
+This can be tuned down to warning by setting the [build](<doc:SwiftBuild>) option `--resolver-fingerprint-checking` 
+to `warn` (default is `strict`).