Updated installation instructions

[ngoiz]

Included explicit instructions to install the release and development builds from source.

I haven't made any particular references to xbeam or UVLM releases, only a checkout from master is required. If you are happy we leave it as is.

The other option would be to have release tags in those repos and include them in the instructions, as is done with SHARPy.

[ngoiz]

I'll make the following updates prior to merging:

• Include instructions for release builds of uvlm and xbeam
• Release tags in uvlm Create release tags according to SHARPy version UVLM#3
• Release tags in xbeam Release tags according to SHARPy release version xbeam#6

[fonsocarre]

I think we can include here too the addition of tags for the Docker container, so that every version of SHARPy is compiled with the proper release of xbeam and uvlm.

• Build Docker with the corresponding version of uvlm and xbeam instead of the latest one.