Adds TLDR multnest instructions

tutorials/TLDR_MULTINEST.md

+Here is a short version, if this fails refer to the full instructions.
+
+First, install the dependencies (for Ubuntu/Linux):
+
+```bash
+$ sudo apt-get install python-{scipy,numpy,matplotlib,progressbar} ipython libblas{3,-dev} liblapack{3,-dev} libatlas{3-base,-dev} cmake build-essential git gfortran
+```
+
+For Mac, the advice in the instructions are "If you google for “MultiNest Mac OSX” or “PyMultiNest Mac OSX” you will find installation instructions".
+
+The following will place a directory `MultiNest` in your `$HOME` directory, if you want
+to place it somewhere, adjust the instructions as such.
+
+```bash
+git clone https://github.com/JohannesBuchner/MultiNest $HOME
+cd $HOME/MultiNest/build
+cmake ..
+make
+```
+
+Finally, add the libraries to you path. Add this to the `.bashrc` file
+```
+export LD_LIBRARY_PATH=$HOME/Downloads/MultiNest/lib:
+```
+
+(you'll need to resource your `.bashrc` after this, i.e. run `bash`.
+