Add few Eigen3 installation instructions

README.md

@@ -82,6 +82,15 @@ To install `SLEPc` on Debian-like systems
 apt install slepc-dev
 ```
 
+#### `Eigen3`
+
+`Eigen3` is linear system solver and an eigenvalue problem solver.
+
+To install `Eigen3` on Debian-like systems
+```shell
+apt install libeigen3-dev
+```
+
 ## Documentation
 
 ### User documentation
@@ -234,6 +243,7 @@ the way `pugs` is built.
 | Description     | Variable            | Values                       |
 |:----------------|:--------------------|:----------------------------:|
 | MPI support     | `PUGS_ENABLE_MPI`   | `AUTO`(default), `ON`, `OFF` |
+| `Eigen3` support| `PUGS_ENABLE_EIGEN3`| `AUTO`(default), `ON`, `OFF` |
 | `PETSc` support | `PUGS_ENABLE_PETSC` | `AUTO`(default), `ON`, `OFF` |
 | `SLEPc` support | `PUGS_ENABLE_SLEPC` | `AUTO`(default), `ON`, `OFF` |