Update the packdiff readme

Author: adithyaov

README.md

@@ -1 +1,21 @@
-packdiff diff streamly-process rev1 streamly-process rev2
+# packdiff
+
+**Usage:**
+
+```
+packdiff diff <package-name> <rev1> <package-name> <rev2>
+```
+
+**Limitations:**
+
+Packdiff uses the hoogle file created by haddock to generate and compare the
+difference between multiple versions of a package
+
+1. The API for modules in the `other-modules` sections is not generated or
+   compared.
+2. The re-exported module is just considered a re-exported module. The API of
+   the re-exported module isn't merged with the module that re-exports it.
+
+The 2nd limitation might end up falsely reporting a diff even if the diff does
+not exist. In our use-case where we have manual intervention this isn't a
+problem and does the job well.