Updates a few details in Contributing.md

* Adds installation of `mitmproxy`. I was getting this error from running regression tests:
```
Can't exec "mitmdump": No such file or directory at /home/glediszeneli/citus/src/test/regress/pg_regress_multi.pl line 215.
```
* Calls `install-all` in the setup. Without `install-all` the `mutli-extension` regression test failed.

CONTRIBUTING.md

@@ -31,7 +31,7 @@ why we ask this as well as instructions for how to proceed, see the
   cd citus
   ./configure
   make
-  make install
+  make install install-all
   cd src/test/regress
   make check
   ```
@@ -51,7 +51,7 @@ why we ask this as well as instructions for how to proceed, see the
                           autoconf flex git libcurl4-gnutls-dev libicu-dev \
                           libkrb5-dev liblz4-dev libpam0g-dev libreadline-dev \
                           libselinux1-dev libssl-dev libxslt1-dev libzstd-dev \
-                          make uuid-dev
+                          make uuid-dev mitmproxy
   ```
 
 2. Get, build, and test the code