Update README.md

README.md

 # Installation
 
 Download the last RTKLIB version [http://www.rtklib.com/](url)
-an save it in your home directory (/home/user-name).
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+an save it in your home directory (/home/user-name).
+
+Create a directory to build all the source files:
+```
+$ mkdir build
+$ cd build
+```
+
+Compile the source files:
+
+```
+$ cmake ..
+$ make
+```
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