Rename intro file in directory to README.md

Users browsing the directory structure will better understand
which file to read first.

README.md

@@ -228,7 +228,7 @@ Courses | Duration | Effort | Additional Text / Assignments| Prerequisites | Dis
 :-- | :--: | :--: | :--: | :--: | :--:
 [Build a Modern Computer from First Principles: From Nand to Tetris](https://www.coursera.org/learn/build-a-computer) ([alt](https://www.nand2tetris.org/)) | 6 weeks | 7-13 hours/week | - | C-like programming language | [chat](https://discord.gg/vxB2DRV)
 [Build a Modern Computer from First Principles: Nand to Tetris Part II ](https://www.coursera.org/learn/nand2tetris2) | 6 weeks | 12-18 hours/week | - | one of [these programming languages](https://user-images.githubusercontent.com/2046800/35426340-f6ce6358-026a-11e8-8bbb-4e95ac36b1d7.png), From Nand to Tetris Part I | [chat](https://discord.gg/AsUXcPu)
-[Operating Systems: Three Easy Pieces](coursepages/ostep/OSTEP.md) | 10-12 weeks | 6-10 hours/week | - | strong C programming | [chat](https://discord.gg/wZNgpep)
+[Operating Systems: Three Easy Pieces](coursepages/ostep/README.md) | 10-12 weeks | 6-10 hours/week | - | strong C programming | [chat](https://discord.gg/wZNgpep)
 [Computer Networking: a Top-Down Approach](http://gaia.cs.umass.edu/kurose_ross/online_lectures.htm)| 8 weeks | 4–12 hours/week | [Wireshark Labs](http://gaia.cs.umass.edu/kurose_ross/wireshark.php) | algebra, probability, basic CS | [chat](https://discord.gg/MJ9YXyV)
 
 ### Core theory