Update Readme.md

closes nmaier#6.

Readme.md

@@ -8,8 +8,21 @@ Requirements
 * [M2Crypto](http://pypi.python.org/pypi/M2Crypto) (recommended)
   `easy_install -Z M2Crypto`
    * Windows users must also install valid OpenSSL binaries into their (Python) path. Copying files to the corresponding M2Crypto directory under Python\libs\site-packages will also do the trick
+   * Mac users might need to install `swig` as it is a dependency for M2Crypto. Easily done via `brew install swig`.
 * OR openssl binary within the path
 
+Install xpisign.py itself
+---
+Use pip (or easy_install or whatever):
+```console
+pip install https://github.com/nmaier/xpisign.py/zipball/master
+```
+
+Or a particular version (tag):
+```console
+pip install https://github.com/nmaier/xpisign.py/zipball/<version>
+```
+
 Usage
 ---
 `python xpisign.py -k cert.pem addon.xpi addon.signed.xpi`